I tend to go by looks....and if somebody pwns me with one, I tend to assume it's awesome and try to get one asap.
Movepool affects it, too.....if I suddenly find out my pokemon has crap variety, and I have 2 pokemon that I like doing the same job one pokemon I really like can do....I usually ditch them and go for the good one.
Overall, it's got to be Aesthetics, though. Sprites don't half make a difference, which is why I wasn't that keen on Milotic in Emerald (sprite is a bit crappy, Gen IV does it more justice)