25 is very functional. It's my going-out bag, plus can go from a day in the city to an evening party. 95% of the time my 25 is cavernously huge to me, even with everything I need.
I stuff a change of shoes, sweater, wallet, iphone, cosmetics case, hat and scarf in there to go out and even then it's not stuffed! A lot bigger than it looks, and I think for me this size is really versatile.
I'm getting a damier 30, though, for those times I really need the kitchen sink-- like traveling.