I used to switch it up quite a bit, but the last 3 or 4 trips I have just used Alamo and been very happy. IMHO, Budget generally has older vehicles, and they didn't seem as well cleaned as Alamo. Our last two trips on Alamo, we've gotten vehicles with 11 miles and 30 miles on the odometer. My last Budget rental the best I could find in the aisle was around 20,000 miles. As noted above, Alamo also has the best "SunPass" fee structure of all the MCO rentals. It essentially costs your $8 per vacation to not have to bother with a sock full of change for the toll roads.
Basically, though, you can't go wrong with either. I'd definitely stay away from the smaller rental operations at MCO, though - we used one of those once and it was a nightmare.