Welcome!
I will try to answer your questions to the best of my ability.
First of all, it's not 3.5i/d, it's 35. The engine sizes are 3.0L twin-scroll turbos. It get's even more confusing with 50i (4.4L turbo).

Weight: It's a known fact that the diesel engine is quite heavier than gasser. I remember reading an auto-review for the 335d (sedan), and they mentioned something like 300lbs difference in weight. These are exact same engines in X5s.

Softer: I would not be surprised if diesel is softer, though I did not notice that during test drives of both. Could be softer after all due to weight.

Body lean: Due to weight, may be more body lean for the diesel.

Acceleration in higher speeds: at 80mph, that's where the diesel shines. RPMs are at 2200, so acceleration to 90-100mph happens pretty quick without downshift. A very nice feeling..
What happens at 130mph when RPMs are at 4000? I don't know, never drove it that fast!
In other words, torque drops rapidly after 3500RPM on the diesel, but then, no-one drives that fast all the time.
I drove both versions a few times, and found the diesel to be more fun in town and on freeway, due to insane torque in the RPM range where people "live": 1500 - 3000.

Hope this helps.