It will depend on how much you like nature. I've been to the falls. They were cool, but at the end of the day you are looking at waterfalls. I've seen the Niagara Falls twice so to me this wasn't that big of a deal, still cool though. Of course if you don't plan on being in that area ever again it might be worth it to go to the falls. You can go to Rio any time.... As far as having a good time though, I would head to Rio.
For hotels, look into:
Copacabana Rio Hotel
Tulip Inn Copacabana
Rio Othon Palace Hotel
Royal Rio Palace Hotel
None of them are in Ipanema but close enough. If you like walking it would be a 15 minute walk, or a 4 minute cab ride.