Both the Steel Eagle and the Big Guy are great machines.
How many gpm's are you running?
Are you using hot water?
I prefer tires over casters because if there are rocks or any type of debris out there, it jars the caster and gets you off-course a little bit, with the tires, you just keep on going. It is weird but with the Big Guy or my Steel Eagle surface cleaners, the castors are large enough and with the tires in the back, rocks are not that much of an issue with the front caster.
I had a 24" surface cleaner with 4 casters and it seems like if there was a rock out there, it would get caught in the casters. We would use leaf blowers but sometimes you don't get all the rocks and nails out of the area.