I haven't sailed on Britannia only Azura. The reviews i have generally read, and the feedback from past travellers is that the britannia doesn't have any lifts apart from the middle of the ship. With the Britannia being a large ship i would find this quite irratating, and apparently the balconies are smaller if you are thinking of a balcony.
Azura i can say is lovely, the outdoor space is very well laid out, very spread out, and seems to have a lot of space, the sea screen outside is pretty special too.
Whatever you choose i'm sure you will have a fab time!