Skip the rental and use B-More's light rail system. It stops right next to the convention center. Also, if you're staying at one of the BWI hotels, they have shuttle service to get you to/from both the airport and the light rail terminal. This is what I did this summer, stayed at the Townplace Suites by Marriott at BWI for half the cost of the downtown hotels. King size bed, fridge, microwave, and two burner stove top with basic kitchen utensils (plates, cups, pots, pans, etc), free wireless, free breakfast buffet each morning (eggs, toast, sausage, cereal), coin laundry, and it only took about 35 min to get to/from the show.
Then it's Tommy this, an' Tommy that, an' "Tommy, 'ow's yer soul?"
But it's "Thin red line of 'eroes" when the drums begin to roll