Ok, I'll throw my $0.02 in on this one...
I came from the ety camp, being one of the early adopters of the er-6is right when the came out. Quickly fell in love with the sound, and later learned that the er-4s was even better. Needless to say I was hooked on detail and resolution and firmly believed that this was the way to go.
Due to chance I recieved a pair of UM1s and at first listen I wasn't impressed. I also found out that with the addition of a resistor cable (75ohm) and a good amp, the sounded right on par with the etys. The detail was there, they just needed a little help. At the same time I had the chance to audition the E500s at a meet. I thought that they were very nice, and definitely had more detail than other phones in the Shure lineup. I also thought that they had too much bass, and bloated it more than was realistic.
I recently aquired a pair of UM2s which I figured would be similar to the UM1s but hopefully better. Boy was I wrong, but in a good way!!! I didn't need to put the resistor in place to ge the detail out of them. The had really good resolution. I don't know that it's on the same level as the Etys, but it's pretty close in my opinion. They had a very nice warmth that I've come to associate with the Westone lineup. The bass was Very present, and didn't overwhelm the rest of the sound. It is a very well defined set of phones. Oddly enough, I discovered that they didn't improve a lot with an amp. There are some parts that when amped open up a little more, but not enough to make it worth carrying in my portable rig. You would be hard pressed to find the combination of sound quality and price to beat the UM2s, even at full price.
In fairness to the Shure E500s, I did only listen to them for a short period of time. I think I only had about 15 to 30 minute, and a fair portion of that time was A/Bing them with my etys. They are the most resolving of the Shure line, but I think the UM2s are at the same level. The bass from the UM2s coms across as more controlled and comfortable. Mids, as many others have posted sound fantastic, and come through extremely pure.
Bang for buck, I'd totally suggest the UM2s. Front edge of trendy, and new stuff, head for the E500s.
good luck
-Jeff