NVIDIA GeForce GTX 580 - 1.5GB

or

AMD Radeon HD 7950 - 3GB

which one is better at running TERA?

This might give you some indication of how they compare.

http://www.tomshardware.com/reviews/radeon-hd-7950-overclock-crossfire-benchmark,3123.html

However, the extra ram would likely perform better because more textures would be able to be stored. The next HD texture patch comes to mind. In conclusion, in most tests the 7950 came out on top, but really started to smoke the 580 when it was oc'd. Hope this helps!

Cheers

(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.