3 shops in my local area.  2 PADI, 1 SSI.    I have certs from all 3 shops, and positive relationships with them all.  The oldest (whom my positive relationship is due to new ownership and termination of old management) is discussing bringing in TDI, as in hiring an established instructor to come in and train their instructors, and teach classes.   I'm partially through SSI extended range/nitrox right now.  I'm interested in going beyond that, but can't without travel for classes.   Bringing in TDI will change that.  I have zero experience with the org.

Are they generally tighter on maintaining quality of instruction than Padi?   Positives? Negatives?

Better than PADI for sure. But in the technical realm, it REALLY comes down to the instructor. The standards are all very similar, it's really just their interpretation of those standards that is different. Outside of GUE, where personality would really only be the potential discrepancy, you really need to pick the instructor not the agency. The shop picking up TDI, but having a crappy instructor, isn't going to do you any favors. It's real easy to get real dead real quick when you're going into the technical realm, and some of the behavior that is considered "acceptable" is pretty atrocious, yet within standards.

The biggest thing I've found with technical instructors is finding someone who is passionate about doing the dives you want to do, outside of their teaching work. If he's only ever teaching the AN/DP course and never dives for fun at a technical level, you can pretty well guarantee he's not going to be a great instructor, because he's never doing real dives.

If you want to start cave diving, you want to find an instructor that actually goes cave diving, not just sees the same 500 feet of cave with students. If you want to do deep wrecks, find an instructor that actually goes deep wreck diving, because your AN/DP course is going to reflect the skills he uses every time he does a dive for his or her own enjoyment.

PADI does have some excellent instructors, and TDI has some crappy ones, so it's important that you're making a good decision based on the person, not the letters on the cert card.

I have certifications from PADI, TDI, IANTD, PSAI and even one from RAID I think. They are different because I sought out my training based on the instructor I wanted to learn from.

TDI started out as a tech agency somewhere in the early-mid 90s. SDI, the sport agency, came about later.  As far as I can tell, PADI only added tech when it was profitable to do so.

One thing I like about TDI/SDI (all my rec level training has been through SDI) is that all their course standards are online, unlike PADI.

Here's a bit of background

To find the standards, pick the course from the below link, and once you're at the course page, scroll all the way down until you see the standards PDF link:
TDI courses

My shop has 2 TDI instructions that go through AN/DP. 1 also does PADI tech all the way through (to Tech 50, I think).  PADI doesn't do cavern/cave at all.

I know that no one typically does their rebreather trimix through them due to their courses being unit specific. IANTD is the heavy favorite due to their non unit specific CCR courses.

They force you to retake all the MOD2-3 if you crossover units instead of just MOD1.

That being said I have OC Helitrox, and my CCR Mod1 and 2 through them. I’m currently taking Mod3 through IANTD.

I have certs through PADI, SSI, NAUI, SDI and TDI.  I do like TDI and the way they are setup for instruction.  I had a lot of options in my area for Tech and thought TDI was the way to go over PADI.  I dont think anyone around me teaches SSI Tech.

I did AN/DP and side mount thru TDI. Johnny C is right when it comes to technical diving; the instructor has a far greater impact on the quality of instruction than the certifying agency. In reality most courses are essentially the same, individual agencies might approach things a little different here or there but for the most part it's all the same stuff. As you advanced up the tech tree you might have to travel especially as you get more and more into niche areas of diving. FWIW most of my open water certs are thru PADI, my tech courses are thru TDI, NAUI, and NSS-CDS.

My Cavern and Intro Cave classes were taken through an instructor I found via a friend who had used him for rebreather certification, and he was through TDI.

I found their course materials concise and easy to move through, I loved their eLearning program, and the course standards seemed all-encompassing.

i can't speak for other tech agencies, but I plan on taking my Full Cave through the same instructor and obtaining a TDI card for it. As others have said, it comes down to the instructor more than the agency. My instructor can issue cave certification cards through multiple agencies (NAUI, NSS-CDS, TDI, etc) but I chose TDI as I had experience with SDI's materials and liked them for the same reasons.

PADI does have cavern and cave. There are only a couple instructors certified to teach PADI cave, and they are all other-agency certified instructors so there's no need to do it. However, PADI will not advertise that they offer a cave certification, and haven't advertised it as being available for a loooooong time.

Most instructors won't bother teaching a PADI cavern course though, as they are generally instructors for other technical agencies too, and most people aren't too keen on a PADI cavern card.

PADI probably knows where their bread is buttered.  Almost  every diver I know that cares about anything other than reefs bails on PADI.  I won't even consider them for my tech stuff.

Meh, again it depends on the instructor. Georgia Hausserman is a PADI IE and Tec Instructor Trainer and I've seen her students first hand in the water and they're squared away. Same with Jim Wyatt. If someone rocked up and showed me a CCR card with either of their names on it I'd have no problem doing a big boy dive with them. In fact, I've done big boy dives with some of them, including 8-hour rebreather cave dives with scooters and deco and all the funzies.

Hell, a couple IANTD cave instructors were directly responsible for their students deaths. You'd think with "Technical Divers" in the name they'd never have an issue but alas.... Hell, the IANTD Central America/Mexico/Caribbean license holder divested themselves from IANTD over the agency's response to one death in Mexico.

Pick an instructor for the education you'll receive, not the logo on the front of the card.

I can agree with that. One of the best cave divers I know is a former PADI OW instructor (and a few rec specialties as well) and he's 110% squared away. I'd consider his students to be very capable if he was still teaching.

You forgot about the instructor and instructor candidate in Italy that got a guy stuck into the entrance of a cave and drowned when his gas ran out. It was around the same time as the Mexico incident and IIRC most of IANTD's Eastern European instructor base resigned in protest but the instructor candidate still got his instructor card.

I've known some great PADI instructors.  But the culture at PADI is to do absolutely NOTHING about the bad ones.   I've seen OWSIs at PADI skip most standards, and bs/fake others, watched them get reported by other PADI pros, and nothing at all be done.   That's my worry with getting into dealings with another agency.

I didn't mention it specifically, but that's the other major one I was thinking of when I mentioned "a couple."

For those that don't know the story, a full cave student died on a course being taught by an IANTD ITT. An instructor candidate was also on that dive and part of the group. The ITT certified him as a cave instructor despite the death that occurred. A significant portion of IANTD Instructors, IT's, and ITT's, primarily Polish, resigned or "got quit" due to their opposition of the QA findings by IANTD's investigation. 236 of them signed an open letter to Tom Mount, significantly Krzysztof Starnawski, who suffice it to say, has done some real shit. (seriously, look him up)

Fontanazzi accident

It's pretty hard to make that argument against PADI in light of what went down with IANTD. I'm not saying PADI is the way to go for you, but it brings to mind something about an emperor and some clothes....

Took a nitrox course with their materials/certification. I was impressed. It was thorough, and I had a good instructor who didn’t cut corners. Like others said, the instructor is probably more important than the agency.

Absolutely, but the culture the agency pushes will have an effect.  PADI doesn't give 3/8ths of a damn about anything except money. It's one thing to survey OW students about the experience, they likely don't know what they're not getting.  DM's reporting multiple missing standards, half assed passing, etc being ignored is another thing.   SSI seems to care a fair amount more.   Having no experience with TDI, I didn't know what the culture of the agency is.