To those saying the SM3 is a huge step up from the UM3X, and in particular @search, I did just have my ears cleaned in preparation for my custom monitors, and I have to say that I heavily lean toward the UM3X's sound signature. My room mate has a pair of SM3's and after spending extensive time with both I can give you my 2cents:
The UM3X's were designed for stage performers, and as such, when you listen to audio, I find that they are very true to the source and the imaging is very life-like. Most studio recordings come out sounding like you are in the studio while the album is being recorded. Clarity and accuracy are the strengths of the UM3X's. In brief, the UM3X's do not emphasize any part of the frequency range, everything seems to be equally spaced. As such, the human ear is designed to focus on the human vocal range frequencies, so the mids seem to come out stronger. Many IEM manufacturers combat this with a v-shaped response curve, although a huge v curve will provide a more "exciting" sound to some.
The SM3's, on the other hand, seem to try and manipulate frequency response to give the impression of a larger soundstage. The treble decay is not as good as the UM3X which gives one the impression the music is further away. The mids are more (relatively) recessed as well, but the bass response is a strong suit to the SM3. They definitely seem to have a few db bump in the low-end compared to the UM3X's which some may like. Overall, I think the SM3 was designed more for regular consumer (albeit audiophiles) listening, and as such, has a slightly more v-shaped response curve than the UM3X. I would not call the SM3's treble recessed, just that it does not have the impact or definition that the UM3X's do.
Overall, many people may find that they like the SM3's better. I believe this can be attributed to the larger headspace and the fact that faster paced music can sound more exciting on the SM3's. Let me explain, the volume level can be increased on the SM3's, and because of their more v-shaped response, instruments get placed before the singer, giving more "action" to the sound. In the end, I found that I really prefer the UM3X's, especially after having my ears cleaned, I found I really loved the increased treble clarity and body, as well as the intimate soundstage and imagining.
In the end, you cannot really go wrong with either IEM, but if you have one and you truly aren't happy with its signature, then maybe switching out for the other would be worth a shot. But if you own either the SM3 or the UM3X and you are happy with the SQ, switching to either would not be considered an "upgrade" by any means (unless your on headfi )
That being said, the poll in this thread is ridiculous lol