Flat bottom floats, like flat bottom boats, have a lot of lift on the water, but they ride hard and without a keel, they don't track well and are hard to steer.

V bottom floats, like V bottom boats, ride softer and track much better on take-off and landing. They also steer better in a crosswind. If the floats are sized right, the slightly lower lift of the V bottom is not a problem.

If you want to spend as little time as possible on the water, use flat bottoms. If you want the airplane to handle well on the water, use V bottoms.

Jim