Have stayed at both hotels the last week and like both. Transport is not that much of an issue if you are not travelling peak.
Metro Rio has a bus that stops just across the road from the Sheraton Barra runs from 6am till midnight every 20 mins or so and takes you to Siquera Campos Metro station in the middle of Copocabana for 3 reias (about US$1.60) in about 20-30mins. You can also hop off at the southern end of Leblon.