I agree with tinyman and can tell you that both offer very different sound sigs. To me, the UE900 is definitely more refined (commensurate with its much higher price tag).
Its treble is delightfully clean and sparkly without being sibilant, a big change fro the steep fall off on the SE215s. Bass is much better controlled, goes deeper, and sounds tighter. And while the UE900s may still be considered ever so slightly laid back/warmish, they are nothing like the 215's predominantly bassy sound. Also, if ‘accurate’ (flat?) signature is your calling, blocking a tiny bore on the UE900 (an easy hack) further flattens its bass curve (see http://rinchoi.blogspot.com/2012/10/ultimate-ears-part3-in-depth-analysis.html?m=1 for further analysis in this area).
The UE900 will certainly take your listening enjoyment to a higher level My only advice is to avoid subjecting the UE900s to ‘environmental stresses’. I can’t speak for you but in my world, 4 benjamins for an IEM is an investment not to be taken lightly, and when my first pair of UE900s died, i was determined to do everything possible to have their replacement last as close to forever as possible. As a result, my UE900s don’t leave the house and the SE215 serve as my beater pair, used in the gym and as a hands-free device for my phone and to listen to podcasts while commuting Their mid-centric signature is especially conducive to such applications.
As a happy coincidence, the cables of both IEMs are interchangeable :-). Enjoy!