Although I'm one of those people who've mentioned that the two are similar, they're really only similar in that they tend to the side of warmth while maintaining detail and clarity amidst neutrality, and impart great instrumental separation. These qualities, along with very good comfort for both IEMs, give both the GR07 and the UM3x an edge when it comes to professional monitoring. The similarities end here.
I don't have the UM3x but have listened to it on multiple, extended occasions and can compare it to the GR07 that I have. The UM3x is much more intimate feeling than the GR07; voices are very separated from the music, up close and detailed. The rest of the music doesn't feel very dynamic, although separation and imaging are superb. On the other hand, the GR07 is more laid back in the vocals, but still quite detailed. Personally, I feel the GR07 has greater overall control over the bass than the UM3x, and more detailed in this lower spectrum as well. The GR07 is designed to be well extended on both ends, but tends toward sibilance and has a slight lower treble hump. The UM3x, in my memory, is a bit flatter up top. The GR07 is vented, and therefore will be a little less isolating. Its housings are less bulky, though. Lastly, we need to keep in mind the natural differences in the rendering of sound between dynamic and balanced armature based IEMs.
Both are great IEMs, with the GR07 being a bit more affordable. I used to lust over the UM3x until I got my GR07. I would still pick the UM3x up should I ever accrue some spare change, though.