I say rent a car if you want to explore the Orlando area. The only reason I am leaning towards yes is that you are from out of the country, and 21 (and no kids) so it really depends on if you want to have wild nights out (which you should take a cab for anyway if you drink) or explore the area and how many days you have. If you're going for like 5 days, then no...if you're going for a couple weeks, then yes.
Most any company is fine for a cheap compact car but some of the companies aren't located in the airport and you need to take a shuttle over to them. It's not that big of a deal, but it is time consuming.