I'd say the difference is pretty minimal. Comparing my several month old CX175 to a straight-out-of-the-box CX275s, the latter might have slightly better mids and a touch less bass and treble, but I feel it's closer to manufacturing variances or small differences in seal from fresh vs. used eartips as opposed to an actual difference in tuning.
To me, the major differences between the two are functional. The CX275s has a straight plug (not right-angled like the CX175), obviously TRRS to accommodate the mic. The mic is about halfway between the left earpiece and the Y-split, so the cable slider only goes as far as the mic assembly. Also, the "extension cord" you're referring to is actually an adapter (only a few inches in length) for use with certain smartphones that have the ground and mic signal switched.
I'd just go with whichever pair is easier and cheaper to obtain. Here in the US, that happens to be the CX275s.
