I had the stx26 and it was a great machine. Then they came out with the stx38 and even better. Also has auto sweep which modulates the machine itself. The bad thing about the attachment 38hp stumper is you cannot use your mini to move wood while someone else is doing a stump.
The stx38 is the best kept secret in the business. I have a PJ single axle trailer with a spring loaded drop gate and can just fit both my 525tx Dingo and the stx38 stumper on side by side. My drive around pickup has both a stumper and mini loader on every job without going back to the shop.
I also have a nice remoted 65hp vermeer tow behind that runs perfect. I can't remember it going out at all last year. My guy got so good with the stx that while I was cutting etc and got ready to go back and get the tow behind, he had even 10' wide stumps done.
I just fired him a few weeks ago so yesterday I had the Vermeer dusted off with a tune up at my mechanic for whatever happens this year with my new guy and the small stumper.
But Mango is right as really you should have a big machine and the small one for the great ground speed and manuverability/access they give. The big one comes in handy for stuff like a 15' wide uprooted root plate I have to do this week. But if you have to have one...I'd go the small one and sub the big stumps out til you get a big unit.