I have used both in Orlando and I find them both to be great. Frankly, I would go with the best rate. There are gas stations all over the place before you get on I4 or the Greenway,
you can top off there.
Actually, I prepay the gas with Avis (not a popular option among the many here) so I don't have to face this issue, leaving is depressing enough.
With National, however, if you are an EA member, they will top it off at current local prices (or darn close).
National's web site explains it quite well.
I found the price they charged me at Avis was within pennies of the local station. Hardly worth worrying about.