The back doors are quite narrow - could be a problem getting sprogs into seats, if you can take a car seat with you and see if you would be able to manage, the beauty of an A2 is that the rear seats sit relatively high, this means you adopt a more sit up and bet postition, fine for abled bodied persons but it means there is less room behind the front seats for these large kiddie car seats, doesn't affect a seated person but could be a problem if you have a child in seat behind you swinging legs as the do hitting the back of your seat while you drive and we all know how annoying that can be, my 2 are 12 & 9 and had no problems at all.
Boot space without the spare tyre is really unbelievable, looks a lot smaller than it really is (it's down to the tall narrow shape)
Running costs - you can't really beat it, ok the prius does 65mpg, but not in the real world, the A2tdi will, I never had less than 50mpg from mine round town, you also get Audi build quality, my A4 currently has 140k on the clock and still looks and drives like a new car! A2's built the same standard.
cheers
Mike