Opinions:

35i has perfectly adequate power, and is the least expensive.

35d has lots of torque. Not so with revs or hp, but still lots of torque. I wouldn't worry about reliability, the base engine has years of service in other countries, it is just (relatively) new to North America. Same reliability as 35i in my opinion. Diesel downsides are higher purchase price, some fumes or smell when refueling, local availability of diesel fuel, and some additional engine noise. Drive one and decide how important some of those relatively small issues are to you and to the primary driver. I think one of the magazines just did a comparison test (X5, Toureg, etc, all diesels) and the X5 didn't do so well, as it is just about to be replaced (7 year model cycle).

50i has lots of power. If that is important, fine, but it would be my third choice of these.

Something to remember on diesel mileage (which is very good) is that the payback period isn't so good, considering also fuel price as well as purchase price. You may have some form of government subsidy, tax credit, or BMW sales incentive that helps with the purchase cost of a diesel. Without that, using my local fuel prices, and retail for the 35i and 35d, the payback period for the diesel (to breakeven) was around 6 years last time I calculated it. That sort of investment return wouldn't ever get off the ground where I work.

As the automakers like to say, people will pay a lot of money for economy.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue