It varies, but when I've compared, I've found car rentals to be less expensive at MCO.
Since I also could fly into SFB on Allegiant, overriding factors for me are: much larger choice of flights into and out of MCO and Allegiant could possibly be the worst airline in the country now: ridiculous fees (even $5 to print out a boarding pass at the airport), limited schedule, lousy service and likely the oldest fleet in the US.
I also agree with the post before mine. Check out Tampa. Its not much farther timewise than SFB, usually more flights, car rentals can be less expensive, etc. Also checkout a split...fly into TPA and out of MCO or the reverse. Generally no car drop off charges in Florida.