I have both q5 and q10. I was pleasantly surprised by the quality and function of the q5 but it's really not a fair comparison... the q10 is clearly superior. Typing on the q10 is faster and more comfortable, photos are superior, boot time is faster and being able to swap the battery is a wonderful option.
I'd like to add that while q10 is a head and shoulders winner for me, there are a couple of things that I appreciate about the q5. I love the range in brightness you can get on the lcd screen; I'm often banging away at night in the dark and q5 gets dark enough to be comfortable. Also, the fact that it's thicker and more narrow makes it more comfortable for me to hold. I'd never say that this is a good trade as I'd have to sacrifice my q10 keyboard, but by itself, the narrower device is easier to use with 1 hand.