NVIDIA GeForce GTX 580 - 1.5GB
AMD Radeon HD 7950 - 3GB
which one is better at running TERA?
(Moving this thread to Off-Topic for you! :))
I benched my own little nvidia and ati comparison on korean tera and found the Nvidia cards won when pitted against similar or ATI cards, as a result i'm going to be running 2 GTX 480's ready to SLI the nuts off tera.
K-tera was running max everything and it had no problems at all.
If you really want just one card there is always the superclocked options for the 580 but its better to do the overclocking yourself, if you dont have an interest in overclocking however i'd say just get the 580, and maybe throw in another one for SLI if you want a little booster shot later.
Also i'm running the EVGA cards. Different manufacturers can make the cards in slightly different spec.