As a traveler with limited time to waste, I really benefited from Sixt's Piazzale Roma rental location. It's about one block away from all the other major brands which are situated one next to the other. Why does it matter? Because once you cross the bridge to return your car, you'll be stuck in a major traffic jam, along with all the other tourists. But at Sixt, it's easy in, and easy out.
Oh, the free upgrade to a Jeep and the complimentary capoacinno were very much appreciated, too.