That would depend on the tier, and also what you want to use them for.
Personally, I would suggest Roserade. While Vileplume has higher defence stat, it's mediocre in all of its stats, while Roserade has higher Sp. Atk, Sp. Def and Speed stats.
Roserade can set up Spikes and clear Toxic Spikes by switching in, it also has Natural Cure so it can benefit from using Rest and then switching out to wake up. Roserade would give good team support, or be a good Life Orb special sweeper.
Vileplume would have a niche as a Cleric in NU; it has Aromatherapy, Moonlight and the same typing (which means it can remove Toxic Spikes via switch in as well). Vileplume is lacking in offense, however, and would mainly act as support rather than any thing that could deal a lot of damage.
@ Life Orb
Ability: Natural Cure
Nature: Timid
EVs: 252 Speed, 252 Sp. Atk, 4 Hp
Sludge Bomb
Sleep Powder
Giga Drain
Extrasensory
OR
@ Black Sludge
Ability: Natural Cure
Nature: Calm
EVs: 252 HP, 252 SP. Def, 4 Speed
Toxic Spikes
Sleep Powder
Sludge Bomb
Rest/ Synthesis
NB: Roserade can't set up Spikes and use Sleep Powder in the same moveset, since they're both egg moves
@ Black Sludge
Ability: Effect Spore (if you can get its Dream World Ability)
Nature: Bold
EVs: 252 HP, 180 Def, 76 Sp. Atk
Aromatherapy
Giga Drain
Sleep Powder/ Moonlight
Sludge Bomb