It would depend on the spread between cost of diesel and premium in your market; mine is about $.45 cents a gallon diff now. For me coming from a 4.4i, if fuel prices were on par, it'd be the diesel hands down on cost. But something more like a Jetta TDI wagon (30/42 city/highway) would get better mileage and make that argument more concrete.

I think the X3 from what I remember is more similar in size to your X5 and they have great handling and I read 19/25 for fuel economy which is close to the diesel X5. Plus quite zippy.

If you had the standard 18" wheels on the 2010 35d I would say handling isn't that precise from my experience; I noticed a BIG change going to 20's with mine.

For me it came down to performance of the diesel and towing capability. Mileage was something I wanted to be better and it is; I now fill up each week just by habit and it's about $26 to $30 to do that (I have very short work commute).

It's a bigger car no doubt and at first I missed my E53 but got quite used to this and I'm not so shy about tossing it around now. I don't miss the repairs from my 4.4i however. I'd seriously look at at X3 for comparison and drive a gas i6 in the X5 too.

Cheers!

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2007 328i Sport, Monaco Blue
2011 X35D - sold -Space Grey
2004 X5 4.4i - sold - Kalahari Beige
2001 X5 3.0i - sold - Pearl Beige
2006 325i Sport, Sonora Beige
2000 323i - sold - Sienna Red