At first, I thought that was the only significant difference between the two models (aside, of course, from the inclusion of the remote/microphone), but upon closer inspection the housing for the driver on the step-up RP-TCM125 is slightly smaller.
I don't know if that means the two models have different drivers, but in the specs each are listed as having 9mm drivers.
In reviewing the ErgoFit RP-HJE120 earphones, what impressed was that they sounded decent and compared favorably with many earphones costing $25 to $30.
By that I mean you get a reasonable amount of both bass and detail, and they only tended to reveal their truly budget nature on quieter, more refined tracks (jazz, acoustical material), where they were apt to have a harsher edge. They also have their limitations with bass-heavy material.
I was hoping the step-up ErgoFit RP-TCM125 earphones would have the same sound. However, they don't. The bass is a little more hyped and they just don't have the same clarity. They still sound good for the money, but not as good as the RP-HJE120s. In that sense, they're more in line with the $7I also reviewed. That budget earphone also wasn't a clean sounding as the RP-HJE120s.
On the plus side, I made a few calls with the RP-TCM125s and while call quality wasn't great, it was decent enough -- about on par with what you get from Apple's.
While the ErgoFit RP-TCM125 doesn't measure up to the RP-HJE120 in the sound quality department, it sounds decent enough for the money, is lightweight, and offers a comfortable fit for an in-ear headphone.
At $11 to $13, depending on the color, it's not quite the steal the RP-HJE120 is, but it makes for a cost-effective replacement for Apple's stock earbuds, and the inclusion of the remote/microphone is a plus for people who want to use their headphone as a headset for making calls.