I'm probably not the best person to ask. My bike was more or less scrap when I bought it, and has needed complete rebuild. I've just got it running right, but it has been a time consuming exercise.
I finished putting the engine back together again today. Strangely, I was expecting to have to replace the clutch side main bearing seal, but it turned out to need a new water pump impeller and piston ring ! The mx model has a single piston ring, so I guess wear on the ring will become evident more quickly than with two rings. In this case, the water pump impeller had failed during a very muddy enduro and the resultant overheating damaged the ring, although everything else was fine.
The hit isn't as hard as I was expecting, but the over rev goes on forever. The low end torque isn't as good as my distant memories of my old KTM 125 EXC. I plan to experiment with flywheel weights to tame it down and try to get it to hook up better at slower speeds.
With regard to maintenance, a neglected TM will become unusable quicker than a neglected Honda. But I guess that anyone that takes riding and racing seriously, and are conscientious about bike maintenance, this won't be an issue.
Haven't ridden a jap 125 in recent years, so can't give a comparison about that.