I would choose Bellossom, just because it's cuter, hehe!
That being said, if you care more about stats than how cute your Pokémon looks, then you probably want to go with Vileplume because of what Jofly said. Although that really depends on what moves you plan to teach it.. If all the moves are physical and mostly not poison type, then Bellossom would be better, although this is an unlikely choice of moves!
If I were you, I would look at the movelists for both Pokémon for the game you are playing, and compare them. Here are the move pages for generation V but you can change which one to view by clicking "other generations".
If there's a move you really like that one Pokémon has that the other doesn't, then I would go with that one. If however your movelists are similar, then compare whether most of the moves are special or physical, and how many are poison type. It depends which game you are playing, but in generations I, II and III grass moves are special, and poison moves are physical... If most of the moves you want to use are special moves or poison type (because of STAB bonus), you probably want to go with Vileplume. If you choose mostly physical moves then boosted special defence might be more useful than special attack, so choose Bellossom. :)