If you're already worried about fuel don't touch the 50i. Considering you've had your E53 for 12 years, re-selling may not be a huge concern...? I wish the 35i was already out when I bought. I think it's a good mix of everything, but it wasn't even announced so I had no hope.

12 years in a BMW - very nice - there's hope for some of us yet. In terms of these things running so long that is. Hopefully the electronics don't bug out too quickly.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr