Well - you should do what you want to do. You are 18 so am I right in assuming you are going on your senior trip after graduating high school? (no offense intended if you are a college freshmen) If so, you'll be onto other things come the fall 2010 (collegetime!)
Anyway, given the 5 month time frame I think you should cut. Walking the beach you'll probably be happier with a firmer chest, more tapered waste, and possibly that six pack. However, a lean bulk wouldn't be a bad move either. Just make sure you emphasize the 'lean' in lean bulk.