I would say that Stun Spore is the better option here. You're unlikely to get much beneficial use out of Toxic since stalling for a large number of turns generally isn't a good idea for in-game battles.
You have not provided details for your team nor your progress in the game, so I don't know if this would be particularly helpful in your situation, but paralysis is good for catching wild Pokemon. It raises your catch chance without wearing off over time (sleep) or posing any risk of fainting the Pokemon before you succeed (burn, poison).
However, if you plan on catching many wild Pokemon, you might want to consider getting a Pokemon with False Swipe instead of using Roserade for this purpose. Absol is a good choice because it gets access to both False Swipe and Thunder Wave, and it has a high attack stat that lets it use fewer False Swipes to get the opponent to 1 HP. In Platinum, you can find one at the summit of Mt. Coronet.
Again, I don't know your personal situation, but I would encourage you to consider any other moves besides these two that you may have access to. Roserade will likely be better off with an attacking move for greater type coverage, as you can do very well in-game from simply having super effective moves at your disposal. You can view Roserade's movepool here.
