I agree with MancRy GB will definitely be mofre comfortable for you and your family. The seat pitch is bigger and the service will be different.
As for the best place to sit, to be honest with two small kids I would opt to sit towards the back, as toilets on the (very long) A321 are near the rear galley and on a 5 hour flight you might find yourself having to go back and forth to the loo many times to change nappies/take the elder to the loo etc and the last thing you want is to get stuck behind a trolley! Besides cabin crew will be closer to you if you need them and you'll see they will try their best to make you and the kids comfortable.
This is just my personal opinion, other people might think differently but I always see parents having more difficulties when they sit towards the front.
I do not know anything about upgrades for GB unfortunately, sorry I cannot help with this!
FBW