11.5" barrel 5.56 & 7.5" barrel 5.56 builds. Both with standard AR-15 carbine buffer tube. Carbine length gas system for 11.5" and pistol length gas system on the 7.5" build.

Should I run your standard AR-15 buffer or an H3 buffer in these builds?

Also, carbine flat wire spring good for both as well? If not, what springs?

Interesting timing.  I just sighted in and ran a few through my new 10.5" build and it ran flawlessly.  Standard carbine buffer and standard spring.  PSA bolt carrier.  Both the 55 grain Hornady SP's and the Speer 70 SP's cycled and ejected 3 - 4 o'clock.  I couldn't ask for more but wouldn't mind trying a heavier buffer once just to see.  Any AR should run with standard buffer/spring.  Tweak as you will from there.

I run H3s and standard carbine springs in everything short of my one 20" rifle. Always perfect. For honesty and accuracy, I have one I assembled which is a bit heavier than a 3. Just the way it turned out.

No experience with anything smaller than 10.5" but on mine, but it was ejecting at 1 o'clock routinely. I'd gotten an H3 and it helped but not much so I emailed Sprinco and the guy said to get an Orange spring to use with the H3. Now, it ejects at 4 o'clock every time, is a b*tch to pull back because of the orange spring, but it works every time. I just bought an upgraded 11.5" upper to replace the 10.5"on the same lower, but been so busy I've only looked at it and put it back in the box. Sad days. The new lower has an adjustable gas block so I'm looking forward to less recoil with that and will see how that works with the orange/H3 set up - may need to modify the buffer/spring choice.
As an aside and to help you spend more of your hard-earned scratch, on my 16" I started talking to Rubber City Armory about adjustable gas keys so I ordered one. Man, it is THE thing. It actually has less felt recoil, it's easy to set up, and doesn't require a re-sighting in. Rubber City Adj. Gas Key for the win! YMMV

I run an H2 with Springco blue spring on my 11.5". It runs flawlessly and eats all types of ammo I feed it.  It's a sweet shooter and one of my favorites. It can't hurt to start with a carbine buffer/spring  and go from there if that's what you have on hand. Sorry I can't help you with the shorter pistol but I would say  that one would probably need the most tuning.. You will know if you need to change anything. Edited to add that you will get lots of different answers. What works for your pistol might not work for others. Take it to the range and  then go from there. Good luck and happy shooting.

As a sidebar, how does this work? Just venting gas into the receiver?

I believe it does, but having to clean more often is a fair trade for how it shoots. I'm sold. If you need to adjust it, take the BCG out, adjust, shoot. no dismantling required.
Link to the product on RCA site: https://rubbercityarmory.com/product/rca-adjustable-gas-key/
Link to their FAQ page: https://rubbercityarmory.com/faqs-policies/
This is from a 2019 ARFCOM thread about the RCA gas key: https://www.ar15.com/forums/ar-15/Rubber-City-Armory-Adjustable-Gas-Key/12-739340/

From what I've seen, their products are highly respected, the customer service is outstanding, and when you call you'll get a shooter who knows what the H he's talking about and will give you accurate information. Right tool for the job? Yes. Firm believer. That said, I don't build guns for a living, nor do I plan to build a lot of ARs in general. Thus, I did not want to pony up for a proper gas key staking tool for the 1 or 2 guns I'd install these on. The guy at RCA told me to send them my BCG and they'd do it correctly and they did: It's properly sealed, used (O.C.K.S.) Ned Christiansen Optimized Carrier Key Screws, which are way better than what it had when I'd sent it, and it was properly torqued, then staked. Regardless if they do it or you do, $49.99 is money well spent IMHO.

Interesting. Less worried about filthing up the innards (it's a gun, that's a natural state ) but I'm thinking more an adjustable gas tube on my N-23 build, if needed.

Thanks for the info!

I suppose I'll start with rhe H3 buffer & kak xp spring since I have them. Worst case, I then try carbine buffer and spring

H2 buffer and Sprinco Blue or standard carbine spring on my 11.5"

11.5's with H3 run fine. If you have the means, try adjustable gas and H1 on an 11.5, mine went from ok to awesome.

I thought the buffer weight has to do with the mass of the barrel, not the length?

My 10.5 Noveske barrel has a .080 sized gas port so she's a bit over-gassed. With Kaw Valley's 5.4oz H3 Buffer and a JP Enhanced buffer spring she runs beautifully and ejects XM193 @ 3:30 and juiced up IMI ammo at 3:00...

I run the same buffer and spring on my 10.3" Aero Precision barrel that has a .070 sized Crane-Spec'd gas-port.  All ammo runs smooth from 3:30 to 4:00 ejection port times.  All 14.5" and 16" barrels run a standard Carbine Spring and H2 buffers... They too are smooth and eject the same patterns.

I also have an adjustable Odin Works Buffer with the tungsten and steel weights that will take it from 3oz's to 6oz's in weight if I remember correctly. I take it to the range with me when setting up a new AR but I haven't removed it from it's package because I haven't had the need for it yet. The right buffer/weight and spring combo will handle most cycling issues.

Have a 10.5 and an 8.5 in .223/5.56.

H2 buffers and std. carbine springs in both of them.  I only shoot 55 grain FMJBT thru them.

Zero issues with both from the first mag. onwards.

H2 and Blue in my 11.5" is perfect unsuppressed. A little over-gassed suppressed but I have an adjustable BCG that dials it down to 3-4 o'clock.

May try out a Red spring because, for some reason, I don't fully trust the adjustable bcg.

I pretty much run this on all my ARs. (16in/11.5in) they eat everything ejection on all is at the 3/4oclock position and have run the shittiest most underpowered trash ammo with no issues.  I thought there was no recoil before the switch, now they really feel like toys especially 223.  This combo took a way over gassed Daniel defense that was ejecting at the 1oclock position and made it a joy to shoot after.

The adjustable gas key doesn't vent into the receiver. it just limits how much gas can flow through the key. the same concept as adjustable gas block, but the gas flow is limited further
"downstream" of the path the gas travels. When the pressure spike from the round passing the gas block hole occurs there is a bit of resistance from the existing volume of air the flow of gas needs to overcome to travel the length of the gas system. If any point within that gas system there are bottlenecks, then those areas of restriction will limit the volume of gas that can travel through the system. I'm sure the adjustable gas blocks and keys will incur a higher pressure spike when the flow is restricted, but we are dealing with the golden age of metallurgy.... at least the manufacturer's of these parts will "hopefully" apply the proper materials in their design and construction....
FROM THE RCA ADJ GAS KEY PART DESCRIPTION BOX: Pre-Hardened 4140: 28-32RC
https://rubbercityarmory.com/product/rca-adjustable-gas-key/

Geissele's recommendations

Was curious about this as well....doing a 10.5" pistol build, the standard PSA buffer seem too light. It's also the noisiest buffer I have, can here the weights sliding around when moving the pistol around (more than normal it seems). Was thinking about going with a H2 instead

Not really. Go as heavy as the gun will allow reliable functionality. With a 5.56 in carbine and shorter, that's typically H3. Never saw a need for anything than a basic issue carbine spring.

I shoot a Geissele 11.5 Super Duty suppressed and un-suppressed using their H3 buffer and Super 42 spring and it works perfectly.