I have both and I think it depends on what you want it for and what you'll do...if you are just looking for a cardio prop, then I would get the rebounder otherwise I'd get the BOSU.
I only use my rebounder as a cardio tool, but I use my BOSU for some cardio (not alot), for my abs, and substitute it for my stability ball sometimes. In my opinion, the BOSU is a more "functional fitness" prop where the rebounder is more strictly for cardio.
I use my BOSU more then my rebounder and part of that is because I vary my cardio alot. It's really a personal preference.
Jo