I would stay at the JWM because you already have the deal. The Copacabana Palace is priced given a large "status" premium. You can easily dine at their superb Italian Restaurant, Cipriani, and have all the experience you'll desire, because the rooms are nothing magnificent.

As for the two Sheratons, if you're looking to score something with FA's stay at one of them, because they're both overrun with them. If you want DL FA's however, stay at the Intercontinental. Otherwise these properties have nothing special to recommend them.

The views from the JWM are wonderful. I still have one I took while there a few years ago as my screensaver, even though I live in Rio.