My experience with MH and TNF is that one would wear the same size in both, but MH is more fitted-to-the-body, along the lines of Arc'teryx. The caveat here is to understand what body shape it's fitted to. Basically, both MH and Arc'teryx are cut more optimally for leaner, more muscular torsos like what you would see on climbers (but not so exaggerated as on a bodybuilder). The current TNF cut is more tolerant of body faults, but is not quite as high-perfomance (read: more excess internal volume).
If one has body faults (that is, if you are something other than lean, muscular and "defined") but desire to wear MH or Arc'teryx, you may have to move up a size. However, it is probably better in all cases to wear the right size with the right fit, as opposed to compromising one way or another.