I had the E900 briefly, very nice phone. Very sleek, lovely design, and very compact. Its more of a phone than the W810i, and is predominantly for calls/texting IMO. But it has a very reasonable 2mp camera, and indeed a good mp3 player.
The other thing is the E900 will shoot better video that any of the current batch of SE phones, the video on the K750/W800/W810i is pretty shoddy in fairness.
I'd choose the E900, something different.