I just started a Pokémon Platinum nuzlocke and I have an Eevee but I don't know if I need to eveolve it to leafeon or keep my roselia so it can be later a Roserade.
It is true Roserade has less weaknesses, so that could be helpful, plus it's dual typing so it can learn a larger variety of moves than Leafeon. Though Roserade has higher special attack and defense stats, and Leafeon has higher attack and defense stats. If I were you, I'd choose Roserade.