I should start by saying we haven't offered on either yet, so there's a good chance we'll get neither!
We currently live in a small city outside London, and we love it here. DH commutes in daily - train journey is under 30 mins. I'm a SAHM, DD is 5 and at an Outstanding primary school that she (and we) love. DS is 3 and at an Outstanding nursery down the road from DD's school, at which he's pretty much guaranteed a place next year thanks to the sibling rule. We've accepted an offer on our current house, and our chain is complete - they're just waiting for us.
House A is on the outskirts of a different town, about 5 miles away from where we are now. It's huge - 1000 sq feet more than we have now, and their asking price is £15k under our maximum budget. I absolutely love it - it's pretty much my dream house, or as close as I'll get to it in this part of the world with a limited budget. It's on a quiet street near a river and a nature reserve.
However. It's at least a 20 min walk to the station - which is a stop further down the line then we are now. DH's train journey would be closet to 40 mins, and with that and the longer walk he'd never see the kids before bed - which he loves to do. He leaves very early in the morning so is gone before they're up - he'd have an even earlier start if we move here.
The local primaries are fine, so we could move DD - but if we decided not to (because her current school really is amazing), I'd be driving the school run at peak times on a busy commuting road every day - 6 times a day for the first year while DS is at nursery.
The local secondary is Outstanding, and less than half mile away.
A smaller factor to all this is that I'm very active in our local theatre which currently a 20 minute walk away. Moving here would make this much more problematic as DH wouldn't be home in time to let me get away. Other things like play dates and nights out (we don't have many but enjoy them when we do) would be more problematic - although there are plenty of kids activities and bars and restaurants in the nearby town; it's not like we'd be moving to the middle of nowhere.
Have I mentioned it's my dream house? 
The other house, House B, is round the corner from where we are now. It's handy for school, the station and the city centre. It's a nice house - very nice - and is about 400 sq feet bigger than we have now, so big enough, but not huge. It doesn't put us in the right place for the best secondarys in the area, but the nearest ones are still good. You'd have to work hard to find a bad school round here tbh.
Downsides are it's right on a busy main road - and round the corner from the hospital, so you'll have siren noise at all hours; has only on street parking (which is always tight round here), and I don't love it. Can't imagine spending the rest of my life in it. DH likes it more though. Its asking price is right at the top of our budget.
Well done if you got through all that! What do you think?