This depends on how far through your playthrough you are. If you are early in the game, you can catch a Budew on Route 204 as one of your first team members, and have it evolved before Roselia becomes available in the wild (which is on Route 208, shortly before the third Gym battle).
The main incentive for doing this would be access to Budew for the early-game battles. In particular, Budew can make the fight against Roark easier, especially if you chose Chimchar as your starter. Budew can also help against Barry's Water-type starter if you chose Chimchar. Catching it earlier also allows it begin accumulating EVs sooner, which could provide a small advantage during battles.
To ensure Budew evolves quickly, equip it with the Soothe Bell you get from Cheryl in the Eterna Forest, and make sure it keeps up with your team level-wise. It's an easy Pokemon to grind, as it gets 4x damage against the Geodude in Route 207 (which you can access before fighting Roark) and Mt. Coronet.
I've used both the Zubat and Budew lines in Platinum playthroughs before, so I know from experience that you can get friendship evolutions going quickly if you're smart. Crobat and Roselia are both extremely gettable before you reach Route 208, so I can't imagine many drawbacks to catching Budew early in your playthrough.
If you're already some way through the adventure, though, then just catch Roselia. There are no penalties for using Pokemon that have already evolved.