I have a CK20HST and KK 48" tiller, bought last summer. Probably 30-40hrs.of use.
Only thing that bogged down the CK was some VERY heavy wet clay, just slowed way down with the HST in the wet spots so engine rpms stayed up. soon as you get thru the tough spots back to normal. Main thing I look for in determining speed ,is how fine I want the soil to be, and if I'm going to make multiple passes.
I picked the KK because its the heaviest for its size, which means it bounces less in hard pack, you and the tractor take less beating. And if you plan on doing a lot of work with it, it is gear drive which will out last chains many times over, all this for less money than any competitors.
Another thing ,if the TSC store has had one in stock for any lenght of time, you can probably negotiate a lower price with the store mgr. I have 6 TSC within 45 miles,3 had my tiller, 2 mgrs didn't want to negotiate, last guy was happy to get it off his inventory. Saved $200.