kiwitakingflight

Why don't you use your Mariott Points + Status to do a status match with SPG + points transfer. Then you can stay at the Sheraton in Leblon.

As someone who lives 50% of their time in Rio, I would recommend this hotel as it is close enough to everything but also an exclusive "haven" away from the craziness of places like Copacabana once your day is done. It even has a private-ish beach and an amazing club lounge with awesome staff. It has good security and during the day it is not a problem to walk between the hotel and Leblon/ Ipanema - but just as easy and very inexpensive to take an Uber (which you really should do at night anywhere in RIO if going more than a few blocks.)

Alternatively if you want to be more in amongst the action I would recommend Ipanema. Hotels are not so great for their price, but the area makes up for it.

Barra is very boring and residential, plus a very long way from Zona Sul which is where you will spend most of your time. Plus traffic in RIO is some of the worst in the world so you will loose a lot of your time to it if commuting between there and Barra. Copacabana is really not the nicest place to stay.... a lot of homeless people passed out from drugs on every street corner, loud traffic at all hours, prostitutes plying their services and noticeable petty crime on the streets. The quality of bars and restaurants there is also not so great... and that famous beach is not the best RIO has on offer. Hence it depends on your sensitivities.

Hope you enjoy your trip, don't get too freaked out about security just try not to look like a typical "gringo" tourist (aka easy and obvious target) and be sensible when planning where you will go and how you will get there.