As for the game, I'd prefer something exclusive, specially tailored for the 3DS. The 3DS is powerful enough (possible of even running the RE5 engine) and the cart size isn't exactly an issue.
They're probably just using the
MT Framework Mobile. Technically the same engine but ported for mobile platforms. It's the same thing that runs SSFIV 3D, RE: Mercs 3D, Monster Hunter Tri G, and UMvC3 Vita. I doubt it's powerful enough for MT Framework v1, which is what RE5 runs on (same with Dead Rising, Lost Planet, and DMC4).