I've had both for nearly 3 months until I decided to sell the UM3X.
The bottom line was basically the sound-stage.
While I could enjoy UM3X dryer and more pronouced mids, it was also what dragged me away.
It was the feeling of everything being so close to you, after listening to Metallica's S&M with the SM3, it was easy to point out the flaws with the UM3X.
Besides that, I do believe the SM3 has the edge on the highs with the right tip, and the bass is at least...