I have both UM2 and ER4P/S. The ER4P seems pleasing at first, but as I get further into each listening session I become less and less satisfied with it. The 4P version is rolled off at the very top and bottom, with a solid emphasis on the midrange. Even with EQ'ing or using the S version, there is still something very unnatural about its decay which lends the sound a very cold and overly-digital character.
In contrast, when I first put in the UM2 my reaction is a bit less positive since the detail isn't right in my face and it sounds a bit dark and muddy, but something about the overall balance and sound quality (with a nice natural presentation and decay) that makes the UM2 sound better the more I listen to it. Stock it has too much bass with almost every ear tip I tried it with, so I did a subtle custom EQ to tone down the bass and bring out the mid-highs. The UM2 in particular sounds great with a Nano and the treble boost "EQ". I find the Shure clear flex tips work best for me, though the gray soft flex tips are close (with more bass but too shouty/glaring on the highs for me).
In an extended listening I vastly prefer the UM2, largely because of the natural headphone-type sound quality vs the rather digital/unnatural and boosted presentation of the ER4Ps, especially in noiser environments where IEMs make sense.
Neither is perfect, and I'll probably keep both since I got a steal on the ER4P (Studiophonic rebranded for $120 new off Amazon). But if I were forced to keep just one, the ER4P would be for sale.