If you ever want to take it on a lake (or even very very tame salt-water applications.....like really really close to shore in the salt-water) then a reasonable semi-V hull like I have is best.
If no waves ever expected (such as rivers only), then the flat-bottom would be fine. Flat-bottoms are usually a bit cheaper too, all things being equal.
I like the steering on a semi-V versus the flat-bottom, however (the V is nice as it digs in a bit on turns)
One problem with flat-bottom boats, is that since they are built for mostly non-wave environments, they don't have much free-board...could be a problem if you have a motor on the back, and you weigh 200lbs....I've seen maybe 3 INCHES of ACTUAL free-board in a situation like that....a bit dangerous.
Keep in mind that on the Harrison RIVER, where it opens way up going around Kilby where the bay is that 2' waves are not uncommon....no fun in a flat-bottom boat.
If you go in any lake of any decent size then I'd forego the flat-bottom boat.
Note: tiny boats, such as 7' or so, seem to NOT have semi-Vs since, I believe, they would tip side to side too much. But a boat 14'+ shouldn't have that particular problem, at least not nearly as much as a real tiny boat. You see huge "V"s on 25' boats, but not the same (18 degree) "V"s on 9' boats, for example.