Britannia is bigger than Ventura and as it is much newer is more luxurious.
Food was very similar to Ventura but did have a snack area by the pool where you could help yourself to sandwiches, wraps fruit etc which was great for children.
The pools on Britannia are all outdoors unlike Ventura. Can't really comment on kids clubs as ours are older so only went in a couple of times for an hour or so.
France and Spain was good. Went to A Coruna and planned our own excursion to the lighthouse then a nearby beach using local buses and saved a fortune over an organised trip. Had a good walk round the town and found an excellent place for tapas. There is a play park right by the cruise terminal.
La Rochelle looked lovely but we made this mistake of paying for an excursion to Il De Re. It was very rushed, we felt herded about and didn't have any time just to wander.
Guernsey - organised with Guernsey Cycle Hire to collect bikes at the port. Excellent friendly service. Rode round the north of the island stopping at beautiful beaches for lunch and ice creams. Left the bikes locked to the railings at the port when we were ready to leave.
We saved so much by looking what excursions are on offer then planning them ourselves. We are by no means seasoned travellers but it was all very easy using the internet and Google maps.
I prefer Britannia to Ventura mainly because it is much newer and more modern.
Any other questions just ask.