Based strictly on the two builds you list, I would get the 50i because it doesn't have the third row, and doesn't have the Tech package.
Actually, I'd get the 35i, but you aren't giving us that option 
This analysis is very much based in today, however, with current fuel prices and the options available right now. If you expect to own the vehicle for 5 years or so, I would build some scenarios based on what you expect the market for cars and fuel prices to be doing five years from now. Fuel will cost more, and diesels will be more attractive relative to large gasoline engines, from a perception standpoint as much as from a cash cost standpoint. So while it may be close to being a wash today, it may not still be in five years (depending on how you assign values to various factors).
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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
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