I test drove both and chose the 3.0si because the 4.8i wasn't worth the extra $8k+ in my opinion. On top of that I would've skimped on some options. It certainly had a lot of go to it but that's where it ends. The difference in fuel economy and the premium wasn't worth what I experienced. They are essentially the same car except the engine obviously... and I'll be hauling a kid around anyway.

Now if it were the 3.0si vs. 3.0sd, I'd pay up to $4k for the difference. Better fuel economy too.

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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