I find that flat bottom floats tend to skip on landing and that v-bottoms land more smoothly.  I use flat-bottoms on smaller planes that one does not taxi much, but just gooses the throttle and commits to flight, and the landings are more of a plop than a smooth swish. 

Here is a link to randy linderman's website.  It is the home of the Chuck Cunningham article that really congealed everything you need to know about floats.  Link