I haven't stayed at the JW, but I've had a few stays at the Sheraton over the last couple of years and until recently lived just a few blocks from the JW.

I'd pick the Sheraton: the pools are nice and it has a resort feel. Great lounge and the plat breakfast at the lounge is actually superior to that of the main restaurant.

Beach is polluted though, and the location is a little isolated but in reality a 5 min cheap Uber ride from all the hotspots in leblon.