Oh MAN I could list 50 reasons why the T10s are better that the se215s! I'll give you a detailed breakdown of preferences so I don't mislead you.
Here is some background to see if your tastes align:
I like any music aside from country. Mainly electro house/orchestral. Some of my favorite artists are The Glitch Mob, Two Steps From Hell, Savant, and Lindsey Stirling. I like legit sub bass rumble, powerful mid bass that doesnt muddy anything up, and mids and highs that portray sufficient detail, but don't dissect the music (I am looking at the BAs here, like Westone W3. Those were excellent, but not if you wanted to truly have fun with your music).
So, some comparisons (I still have both) RHA T10 (Double flanges, treble filters) vs Se215:
Sub bass (Test songs: Dark Ages by Two Steps from Hell, X-rated by Excision (bass freak song))
T10 - Powerful. Impactful. Unstoppable. Brute power here, truly makes your head rumble. Supremely satisfying. Can tone it down with si gle flange tips, or you can turn up with the bass filters
Se215 - Weak, I would go so far as to say nonexistent. I cranked up the volume of Dark Ages all the way, and it was rumbleless, pitiful. It sounded so flat and dull, I may have yawned a few times. Lame.
Mid bass (Test songs: Greyhound by Swedish house mafia (2:20-3:20))
On both, it was very lively. T10s sounded cleaner, while se215s sounded warmer, but muddier.
Mids (Test songs: Shatter me by Lindsey Stirling, Burn by Ellis Goulding)
T10s were surprisingly exceptional here, especially with Burn. The vocals just sounded so....intimate, and fluid. Very clear as well.
Se215s were somewhat muddy, which took away from the enjoyment a little. They did sound best here in my opinion though, albeit somewhat dull, strangely enough.
Highs (Test song: To Glory by Two Steps from Hell)
Highs on the T10 were a bit more pronounced in general (treble filters though) , but never sibilant. Se215s had a weird fuzzy sharp sound at high volumes. Highs were superior on the T10s, which comemented its superior clarity.
Comfort: both excellent, edge goes to T10i for not sliding out of my apparently alien ears.
Build: both excellent, edge to T10s for steel shells and better feeling cable. Longevity isn't an issue for either (though the Se215 MMCX connectors began acting up after about 18 months)
Looks: T10s. Also, their presentation in the box is breathtaking, and I usually don't get giddy about packaging.
TL;DR
I must say, comparing both sound sigs as a whole, I cound the se215 to be very lackluster no matter what I threw at it. The signature was just dull. Lack of sub bass and weak highs are perhaps most attributable to their muddy, dull sound. T10s bring your music to life. Not to mention they are easy to drive! I would rate the se215s a 6/10 for sound and the T10s a 10, but JUST for my personal preferences.
Only benefit se215s have are isolation, and would be better for smaller ears, but aside from that they are quite weak. Though to be fair, they are 1/2 the cost.
My recommendation? T10s (not T10i's, cause T10s have free range cable cinch). The sound is really incredible, even without burn in. The bass is incredible addictive, thee highs clear everything up, and the mids are liquid and very enjoyable. Not to mention the build, fit, and 3 year warranty. I hope I don't sound like I am overhyping, but this is how I feel they are compared to my previous IEMs.