Price, performance, noise, heat, power consumption(idle, load i don't care that much) and manufacturer. As for dual cards i do not need/want any. Single card is enough.
I like the 5770 and 5850. I own 5770, could not afford the 5850 at the time of the purchase. Wish i had one.
As for size, i own a mid tower case, if it doesn't fit, just move the hard drives, problem solved