I'd take the diesel over either the 35i or 50i, any day of the week. You still get more than 400lb/ft of torque, although not the uber refinement of the gas engines.
I must be getting old, though, since when I was considering an X5, I just wasn't happy about the ride quality. You feel every imperfection in the pavement, which is a dealbreaker in TO, where they seem not to know or care how to build and maintain roads.
That would really put me off the X5M, since I imagine its ride is worse than the lesser models.
If I was still in the market for a big SUV, and I could live without a diesel, I'd go for the LR4 over the X5. Or perhaps the GL Bluetec. Massively more interior room and they each ride like a limo.