It comes down to comfort of use and how effective it is when cultivating.
With the crazy gear reduction speeds none of them will run out of power. In my opinion, the Mantis and Echo's gear speed is a bit too fast.
When using Mantis, Echo or Honda units, your wrists will get fatigued pretty quick due to the handlebar design. If you hit a hard patch of soil the unit will jump straight up. The unit literally hops up and down like a poodle on caffeine. The gear speed is almost too fast since this happens a lot. You literally have to hold the unit down, using your wrists in an unnatural position.
The MM 55 has a unique handle design. The handle grips are a rubber composite that is called a comfort grip. The angle of the handlegrips is extremely comfortable. The length of the handlebar assy put the unit at a 45 degree angle when in use. This puts all of the weight on the tines and is less likely to jump up when harder soil is encountered. If the unit does jump up, the movement of your wrist is similar to using a fishing pole and twitching the jig in the water. It is extremely comfortable to operate. The handles down so the unit can be hung on a wall when not in use.
Ask your STIHL dealer about the 7 Day Satisfaction Guarantee. If the dealer particiaptes, you can Buy it, take it home and try it and if you do not like it you can take it back and buy the Mantis.
My first one has been on loan to Neighborhood for the last several years. I'm tired of tracking it down when I need it. I am in the process of buying my second one for my personal use. LOL