Since english isn't my native language I'm always searching for words to say what I mean, that's why I made the comparison in Dutch.
Basicly what I've noticed is that the UM2 sound a bit muffled compared to the ES2 which has a great allround sound, solid bas, great highs. The midsection is a bit emphasized but not disturbing. The clarity of the ES2 makes it a winner above the UM2 in my opinion.
More on this subject can be read (again Dutch!) here: http://mp3cdbrowser.sourceforge.net/es2.pdf
In which I made a comparison between the 2 westone IEM's using my home rig listening to some of my favorite CD's. I've listened to all CD's 3 times:
- UM2 only
- ES2 only
- UM+ES (swapping them back and forth)