Enterprise Rental Car on Huron, which is walkable from the west-side leafy neighborhoods and from downtown, is worth adding as a rental possibility. They even pick you up. And you don't have to go to the airport.
I live close enough to that location to walk to it and they know who I am, but they unfailingly offer to pick me up when I call in to reserve and offer to drive me back home, when I drop off the vehicle, even though I've never taken them up on it.
We're frequent renters from there, because we prefer to have a nice reliable car with a comfy ride for long, out-of-town trips (just came back from a drive to Washington, DC through the hinterlands of Ohio in the comfort of a PT Cruiser) and not put extra miles on the little Honda Civic Hatchback. Can't vouch for the competitiveness of their rates, but they've nailed the customer service every time.
438 W HURON ST
ANN ARBOR, MI 48103-4206
Tel.: (734) 327-2600
Web Link for that location: