Make sure it's 7075 - T6. Not that 6061 crap. I have a mil-spec one from BCM and one from Damage Industries on my current rifles. Both have been good.
Buffer weight depends on gas port size and gas length. If you're running an adjustable gas block, you can tune the gun to run excellent with a carbine buffer. If you're not, run the heaviest buffer that reliably cycles your weakest load.