The Brazilian side is amazing! Do not miss. Of course the Argentinian side is amazing too.
I stayed at the Sheraton in 2009, and dropped in on the Brazilian hotel when I visited that side. The Brazilian hotel seemed a bit old-world, but I can't speak to its current state. The Sheraton is extremely special because of its location, but otherwise a 3.5* hotel by my estimate. 3 nights allowed extra exploration, but I could have done more with 3 on one side and 2 on the other, rather than 3 nights at the Sheraton and a Brazilian day trip.
The Sheraton does have suites, so if you want to upgrade, that's an option.
I would rather stay at the Sheraton in the park than in one of the 5* places further away.
Get the guided full moon walk if the timing is right, and the jungle tour.