I have been to the Westin a number of times. It is literally 5 minutes from the airport and I am fairly certain there is not a reason to have a rental car there. You will walk to the marina when you leave the resort and take a taxi into town at night if you want to go there. Not worth driving downtown, parking and worrying about how much to drink when the taxi is a lot cheaper than the rental car. Maybe a better idea would be to take taxis while in PVR and then pick up your rental car to go to the SR. Not sure if there is someplace at the hotel but you can be at the airport rental car places in 5 minutes.
PS: if you rent a car don't ignore the collision damage waiver issue in Mex. Make sure your credit card covers it and then think twice about whether you pay anyway as in Mex this is a revenue generator.
This is sounding like a better idea. I figured I would just get the car for a week since both hotels offer free parking (per their SPG sites) but I doubt we'd be using it much while at the Westin anyway. I will definitely be looking into the coverage provided by the various cards I could use to see what I should/shouldn't purchase but if it comes down to spending $20-$30/day extra to not have an hassles, I'd think that was worth it.