SO... I've done my share of ordering girls just like you boys do, both with a guy and by myself. After all, I was a hobbyist before I was a 'hooker'.
I certainly look for girls in a certain size range. I certainly look for a certain quality, meaning ones that use nice hotels, keep themselves well maintained. I certainly look for a certain personality type. I would say that DEFINITELY performance is more important than looks, for me anyway.
I've chosen super hot girls before and one specifically comes to mind. She was visiting, super hot, but performance was blah. She acted like it was a privilege just to see her boobies. No licking of my coochie, no letting me use a strap on, etc... what's left for me to do? Talk? um no lol Performance 0. Looks 10+. Repeat - NEVER. I'd see a 7 or 8 ANY DAY. She didn't like girls and it showed. She liked my $500.
Also, I think I'm a pretty good example of this =) I'm not your typical model type, by any stretch of the imagination. I bring the energy and mood to the session that I'd want a girl to bring, choose my boys wisely, and rock the fuck out of their world when I see them.
Peace.