The letter last week (Feb. 10) correcting The Jewish Standard’s usage of mentsch and mentsches pointed out that the correct spelling/pronunciation should have been mensch and menschen, based on the German origin. The Harkavy Yiddish dictionary agrees, although it does list mentsch as an alternate, presumably in common usage.
I would not presume to know more than last week’s letter writer, but would guess that it’s physically more natural to pronounce with a “t” than without. Curiously, my spell-check corrects mentsch to mensch, but menschen to mensches. It’s always difficult to know whether to insist on literal correctness when it’s contradicted by usage. How often do you correct someone who says “It’s me”?