Mitsumi, you might recall, is the electronics company responsible for Nintendo's SNES controller, the first Xbox pad and many, many others. They're the single biggest unknown name in video game pads, and their design experience is, IMO, completely unmatched.
ALPS, however, doesn't know a lot about pads.
Both companies made PS1 controllers that were identical at first glance, however the ALPS pad is distinctly inferior, and it all comes down to the d-pad. The Mitsumi d-pad is comfortable and pliant, where the ALPS-made d-pad is stiff, with a rougher texture to it, resulting in sore and blistered thumbs with any kind of extended play.
I'm secretly convinced that a lot of peoples' negative opinions of the PS1 d-pad are a result of playing with the ALPS pad: if it was your first experience, you'd have probably disliked it. The problem never really went away either: where Mitsumi's pads would quickly reach a friendly broken-in stage, the ALPS d-pad was always stiff, and you could never really play it long enough to hate it less.