The PK3 bass extends a bit deeper than the KSC75's, and they're about similar in punchiness. KSC75's have a bit more clarity across rest of the range, but perhaps a bit too sparkly for some people's taste. It's kind of like PK3 vs PK2 (which KSC75 somewhat resembles). Either are a fine choice, really, and it'll mainly come down to if the user prefers buds or clip-ons.
I prefer the KSC-75s in terms of sq because it has that extra sparkle(?) in the highs and it seems like it has a little more detail but like others have said, they look weird and they're not the best for portable use compared to the PK3s. Overall, I like the PK3s better, but that's just because with my listening habits, I can use them more often.