Gen 3s have the ability to set them so that only a few select Vins use them. They also have access to wifi so they can get firmware updates. Gen 2s are rated for up to 80amp circuits, so running them at your typical 50-60amp setup, they are operating at much lower than max capacity so dont have any issues overheating, etc that I have read about.
The cable for a gen 3 is thinner than the one on the Gen 2. Some see this as a negative, and some see this as a positive.
Gen 2s power share via a wired connection, gen 3s power share via a wifi connection.
Tesla does not have parts for gen 2s anymore for sale that I am aware of, either.
I happen to prefer gen 2s to gen 3s, but thats because i dont need or value the wifi updates on the gen 3, like the fact that the cable is thicker on the gen 2 (thus less heat), and that the gen 2s running on a 60amp circuit for 48amp charging are no where near their max capacity. They also seem to be made better on the inside (gen 2s) vs gen 3s.
I happen to like new products vs used though, so if I was buying right now and didnt have anything, I would likely buy a brand new gen 3. I am not a fan of trying to deal with missing parts / templates etc on used gear.