Personally, I wouldn't go for a previous rental or even lease return for that matter, at least when it comes to performance cars or cars of similar caliber. Just knowing what my own car goes through, with my spirited driving, all the warranty issues I've had, and everything else, I wouldn't even buy my own car after the fact (lease). Cars on dealer lots with a few hundred miles get thrashed from time to time, let alone a car with 20,000+ on the odometer put on by many different drives. That said, these cars are built to/should be built to take some kind of abuse from time to time given their performance nature, so if it has all the maintenance records, checks out during an inspection, and you're getting a killer discount, I would go for it if you're opposed to leasing new.
2012 Mustang V6 Premium Coupe MCA - Performance White/Charcoal Black