Which should I buy?

I've heard nothing but good things of the TN lowers. Not to mention they offer completely clear lowers.

but why?.....you can get a quality aluminum lower for 50 bucks

If you need to ask why, then you need to understand 80% first.

I thought Tennessee arms are complete lowers?

I would not buy either

My main reason for the plastic over aluminum is the cost of the lower with jig.

The clear is cool

But clear is the weakest of the colors. I would bet it won't make 100 rounds.

There are videos of them shooting it with a full auto upper. It is weaker, yes, but it’s still stronger than you might expect. The biggest concern if I remember right is the fact UV damages it. So that naturally limits its use. I think they are more for safe queens or conversation pieces. I wouldn’t mind having one just to have an eccentric toy.

I have had success with Polymer80.  They are thicker/reinforced in several areas that the Tennessee is not.  I wasn't sold on the Tennessee brass insert.  In theory it is a good idea, but in practice the polymer will be thinner around the top of the receiver to accomodate the size of the brass insert.  The stresses are still there, they just moved to where there is less polymer material retaining the brass insert.

Polymer80 lowers are fun to build and easy to do without a lot of specialized tools.  I have not yet had one break, but I have since moved on to aluminum.

I like all that material at the top of the JMT...

There are those few photos of broken lowers the Poly haters like to share. Is there actually a documented weak point in the lower? Does the buffer tube break in use in the field or when abused in the truck?

I am several hundred rounds into a Polymer80, but those are .223. No sign of stress fractures at this point.

I have a JMT that I have been using with an Aero Precision .300 BLK upper for a while. It has been working for at least 200 rds without a problem. After each range session I check the lower for fractures or wear and haven't seen either.

Haven't tried Polymer80 AR lowers yet, but their pistol lowers are very good. Might try a TN one and give it a shot to see how good that brand is next.

If anyone is afraid of plastic lowers breaking, they need realize how much we depend on plastic products in everyday use. Including the car or truck you drive.

I have a self-designed 3d printed lower made from a material FAR less formidable than what those companies are using. And “printing” is also a much less ideal way to form the lower. Plenty of 5.56 through it now. With 5-6 hunting trips. No issues at all. And this is the first time I have ever build a gun…

My 2 cents.

The early ones especially had problems - ATI had a all polymer variant that was prone to breaking very quickly - it was a near copy of the std aluminum lower.  Then ATI snd others seemingly fixed that weakness by adding the brass/meta reinforcements at the weak points and other polymer manuactures have beefed up the design.  I think Polymer 80s seem to have a reasonable reputation for being all polymer and the polymer/metal hybrids seem to have some good evolutionary steps.

Aluminum is best for durability, Polymer is improving, not sure if its perfected.  I have a few polymer lowers that seem to work fine, but not betting the farm on they are as good as Aluminum.

I have approximately 1000 rounds through my JMT gen 2 lower. No issues.

Milled out this Tennessee Arms lower a couple of weeks ago and built it as an AR 9 with a 16" barrel. This is what happened after about 250 rounds. In comparison, I finished a neon green EP Armory polymer lower that has had over 1,000 rounds through it without issue.

Interesting, not exactly where I would expect the failure point to be.

Both 9mm? Same upper?

Both 9mm. Same brand of upper, both from Delta Team Tactical.

The EP Armory lower seems to have a little more give while the Tennessee Arms one seems more brittle. I emailed TN with the pics and they think the fit between the upper and lower may be too tight, but honestly I can push out the takedown pins with my fingers while on the EP Armory, I usually need a punch or allen wrench to pop those out. TN did say they would send me a replacement when they received the broken one back.

How cold was it when that lower broke? It seems like it was very brittle. I've never heard of a polymer lower breaking in that area either. I'm thinking either it was frozen or a bad batch of polymer.
I've read lots of stuff about polymer lowers breaking, but very few recent stories, yours really being the only recent story I can remember. Most stories and pictures are Vulcan, Bushmaster carbon 15, and first gen EP armory.
Judging by how often EP armory is sold out I guess he sells THOUSANDS! of these things. And when I search I'm still reading stories about the guy who hid his first gen EP under the couch cushions and sat on it and it broke.
I've got a couple EP lowers to play with. The first one I had a PSA middy upper on it and I got around 500 rounds through it and it was still as good as day 1, but I decided I liked that upper so much I threw it on a forged lower. I still use that lower for testing new uppers. Now I've got a zombie green lower that I have a 7.62x39 upper on. I've put a few mags through it. I've even grabbed it to carry for protection in bear country.

Do I think these are the type of thing I'll be passing down to my great grandkids? No. Metal is forever, plastic not so much. Same gos for pmags and glocks.

Can you build a serviceable firearm with a polymer lower? Yes.

My experiencec with the EP Armory lower, which I purchased a couple of months ago, has been positive so far. Like I said, it seems to have more give and flex. In that respect, it was harder to mill out neatly (I used a router based jig) wheras the TN was crisp and responded more like hard wood to my bits. The two lowers really do feel different to me when I handle them side by side. BTW, the crack occurred at an indoor range where temps were comfortable. Even though TN will be replacing the lower, it's hard to trust their product after something like this. I might just use the replacement lower as a trigger test platform.