[cowboyt]

I likewise have both the C3 and C4 systems. Turns out the C4 systems with the 2016 software also support HHT-WIN, and I learned that by hooking mine up to an older car that requires HHT-WIN. Sure enough, HHT-WIN came up, and I could diagnose the car. So, it's less the MUX, and more the software, in that sort of situation.

The MUX matters for the year of car. The C3 supports up to and including at least 2012 cars, maybe 2014. The C4 supports the newer cars as well. Therefore, if your car is 2012 or earlier, just stick with the C3 and you'll be fine. I find myself using the C3 about 2/3 of the time, but I won't hesitate to bust out the C4. Both are good systems for your S55.