"what are the differences between the two?"
About 30% in fare cost. Nothing more. Regular yellow taxis (with the name and phone of the cooperative painted on the rear fender) are fine.
You can do most sights by taxi, easy and cheap. No tour/guide needed.
Pick a clear day to go up to the heights (Corcovado and/or Sugarloaf). Go early.
Info on Rio (things to do):
www.ipanema.com