The UE900 has a rather thinnish and smallish sound, but very nice with good clarity. The 535 is thicker and bassier with a larger sound-stage.
To me, the UE900 sounds best with the tips that were originally on them when I opened the box. Using those same UE tips on the 535 produced a similar brighter mid, but the highs were a little less crisp than on the UE900. I did a little (same) tip comparisons because sound seems to change sometimes significantly with different tips. The 535 was all over the place but the UE900 stayed relatively the same. But again, I found that medium tip sealed better for me. That same tip, however, would likely not block much truck noise so I have to change up because the truck is where I need IEMs.
I mainly listen to R&B, Pop, Rap, and Hip-Hop where strong mids, strong bass, and energetic percussion is important. I listened to some Stephanie Mills yesterday and thought the UE900 was nicer ("clearer" sounding, though less bassy, than the 535). I then played some Earth Wind & Fire (with all the background singing, horns, and other instrumentation) and the 535 stepped up (and out) with the high energy big-boy sound and soundstage. I'll compare them with some Gino Vanelli next (among many others) before deciding whether to keep the UE900 or take it back.