They're not very fun to use in gen II, honestly. I don't think they really got very interesting until gen IV. With Vileplume, you at least get added STAB with Poison typing that can add to your enjoyment, but I suppose in the end, it comes to how you like to play. If you enjoy inflicting status moves on opponents and being a nuisance, both can do that job. Bellossom is slightly tankier, but has less to play with if you just want to go offensive. I wouldn't call either of them bad Pokemon, but there are certainly better Grass types. But this is coming from someone that uses Sunflora for fun in gen II so take all this as you wish, lol.