Deadeye and Calipso, both have good points. I was gonna suggest the same thing as deadeye but I didn't see the question until a month after it was posted so I didn't know if anything I said would be moot or not. If you stay at the Luxor or Excalibur, you can take the free tram to MB. If you stay at the Trop, you just have to walk across the street to Excalibur.
Resort fee vs Rental?
Like Calipso said, you have to consider the resort fees too but when you rent a car, I always buy the loss damage waver. I know, I know its just a money maker for the rental company but if the car gets damaged or even vandalized guess who has to pay for the loss of revenue? Your own insurance or even you credit card should cover damages but the rental company can still charge you, per day for the amount of time the car is out of commission. Just something else to consider.
I don't know if they still do it but the last time I rented a car through Orbitz, they offered a 3rd party insurance that would cover the LDW, and it was only like $40 bucks for 5 days. If you buy it through the rental company the daily charge is as much as the daily rental fee which more than doubles your entire rental cost.