I haven't rented there, but I'd say that before you go back to Logan, consider the Hertz location just off the common in the garage behind the Four Seasons. I think this is probably where the Sheraton gets their cars if they're short. But even renting there I've never found a great selection, but the airport fees are so high at Logan (not to mention the Ted Williams Tunnel toll) that I'm willing to suck it up and rent in the city every time it is an option.