MarkH -
I just purchased a Howard Rotovator3, 48inch tiller for my BX2200. $1400, tho' they can be had for a couple hundred less. I had the same dilemma as you a coulpe months back. The small tiller from TSC costs a couple hundred more than the 5' tiller and I wasn't sure the BX could handle it. Those KingKutters tillers at TSC look well made to me.
My experience w/ the BX+howard 4' tiller in my medium clay soil is that it just eats it up with no trouble at all, even with wet soil. If the BX will handle the extra weight on the rear lift and your soil isn't worse than mine (tough to get a hand shovel in mine when dry) you should be able the use the 5' tiller - no problem.
Tho' the BX is small, it does have as much HP to the rear PTO as many of the bigger compact Kubota's.
Like Steve, I have a Howard Rotavator HR-3, though mine is the 52" version, for which I paid $1350. It goes through hard clay like butter...just fine for my BX.