When I bought my E4, I was already 'comfortable' with the size (or lack of it) of the UX boot space! So AWD didn't make much of a difference to my decision. It wasn't that expensive as an option and is a nice to have, I think.
For anything other than weekly food shopping or 2 weekend cases etc, the rear seats have to come down anyway.
The UX is not really a family car, but it's great for us as a couple. I can still get 2 grandkids 6 and 8 year-olds in the back with their car seats on school pick-up duty once a week - but I wouldn't want to do it every day!
As for AWD performance, I can't say I've noticed it so far - but then we don't (or at least haven't had) a winter like in Sweden - yet!
Regarding mpg, it's all going to be down to how you drive it.