Most (if not all) rental locations at Tampa are right on site (I believe that National and
Alamo are still a shuttle ride away). If traveling with a family, it's not hard at all to save a nontrivial amount of money by changing airports. Even if it's only $50 a ticket, that quickly becomes $200 for a family of 4. One thing in to look into though, is the cost of car rentals at Tampa versus MCO. I know rentals tend to be cheaper at MCO than at Fort Myers. If the cost of the rental is more, then the savings on airfare might be negated. Hard to know...