What GPU features are important for hash cracking?

[xamio]

Ah okay. Would you be able to give an estimation on the following (would help a lot!):

a) GPU 1 (768cores and 850+boost freq) vs GPU 2 (768cores and 657+boost freq) on single MD5 hash
b) GPU 1 (960 and 706+boost freq) vs GPU 2 (768cores and 657+boost freq) on single MD5 hash

Thanks so much!