I owned and sold the MTs. I felt the treble was a little off, but overall a nice IEM. I now own the S4s, and I find them to be the best value in IEMs at their price. I also own the UM3X, and not saying the S4s are on that level. But I can say that once the S4s are burned in properly (they are harsh out of the box, you can check my early posts. I didn't care for them), they sound simply fantastic - my choice over the MTs. Just my two cents. To really appreciate the S4s, I moved to a modded pair of Shure tri-flanges (white), trimming off the top flange and cutting the stalk to even at the bottom. Just great, and as comfy as the gels (I had been using the large).
Anyway, I really feel the S4s, if given adequate burn in (70-100 hours) are a better choice.