We have the Stihl MM55 at work. I like it better than the Mantis, I think because it seems to be better balanced. It's been a while since I've run a Mantis, and I've never had the two next to each other, so I can't tell you more than that....
As for the wheels, ours doesn't have those. I don't think I would want them anyway. The smaller tillers like these you don't till very deeply with, and you tend to pull them backwards then let them back forwards. It's a constant motion, back and forth with them. I think wheels would interfere too much unless they folded out of the way. At that rate, it's easier to pick up the tiller and carry it rather than trying to push it. They don't weigh much.
We got ours last fall, and haven't used it a lot, but enough for me to know that I do like it better than the Mantis I used to use. Not to mention that it's a Stihl, and getting it serviced should be easier than a Mantis.....
HTH.
Dan