Among the hotels discussed here the Marriott has the best location.
I'm also going to be in Rio during the first two weeks of August.
I've been there many times, and I prefer not to stay at the american chains when I go there. They tend to be overpriced and/or have lousy locations. I would not stay at the Hilton Barra if they paid me. It's in the middle of nowhere. The prices for this coming August are pretty reasonable, so I would rather choose a nice hotel with great location and save my points. But if you are concerned about the future of the SPG/Marriott points, and wanna burn them, then stay at the Marriott. Sheraton is a bit more isolated, and that's not what you are looking for.