Both use the same drivers. The difference is that the KSC-75 is coated with titanium and it lends a sparkle to the response. Bass is big, but not overly-boomy. Midrange is great and the high-end has a lot of sparkle. In addition, the KSC-75 is extremely efficient. Simply put, the KSC-75 is the best buy in headphones, period. You have to spend ten times as much to get equivalent performance.
The PortaPro pre-dates the KSC-75 by a couple of decades. For years, it was the finest portable headphone available. With the typically bass-shy (before Mega-Bass) cassette and CD walkmans, the PortaPro gave tremendous bass and slam. It was as good as you could get for the price. Even though it uses the same drivers as the KSC-75, it does not have the titanium coating. In addition, the PortaPro uses the headband and temple cushion to apply much greater force to the ears as opposed to the simple ear clips of the KSC-75. The result is even more emphasized bass, with very little high-end sparkle.
Note that Koss also made the KSC-35. It was the exact PortaPro drivers in an ear-clip format like the KSC-75. Unfortunately, it only resulted in a bit less bass emphasis, but still - no sparkle. Since it was a slight improvement over the PortaPro, and because they basically quit making them after coming up with the KSC-75, it got an undeserved cult status. It is not worth pursuing, IMHO.
As things progressed in headphone audio, the PortaPro did not. These days, they are - comparitively speaking - boomy, with over-bloated bass and not much sparkle or detail.
Buy the KSC-75. Everyone should have at least one pair. I have several. The only downside are those funky silver caps on the drivers. However, Massdrop made a special version that was all black. They look as good as they sound, but either one will sound just as good.
