I like both and have a lot of both. Specfically, six LC+ and perhaps four times as many MTH. It is important to realize though, that most of my MTH I have are Premier (far most expensive) or upper-end RK/Imperial (merely more expensive) than LC+.
So, comparing LC+ to semi-scale, $300 (after discount) range MTH locos, I prefer the LC+ steamers but will not touch the LC+ diesels.
Why? Everything said above is basically correctly. I do not view LC+ as only for kids, however. Yes, DCS and Legacy have many more features and far more things you can control more precisely, but how much do you need? I prefer the simplicity of LC+, and the fact that it uses radio, not the track, for signal prorogation. It gives me all the control I need including awesome speed control for switching (which I rarely do but its fun to have).
I have four LC+ steamers and love them all and they have proved very durable and reliable. I don't buy LC+ diesels because I have two and both have failed and prove to be unrepairable (by me, maybe Lionel could fix them, but I think they have some design defect).
I have some recent MTH diesels in the LC+ price range, such as the recently released RK NW-2. It is a super little engine. Sclae size, great runner. Okay sound. Adequate puller, but not the greatest (it's a surprisingly small loco so it doesn't have weight on its side like larger diesels). I would have preferred to pay $50 more or thereabouts for smoke, though.