I would suggest the fuel increment is no more than 1 mpg.
I find the handling slightly better with the x-drive plus there is the traction improvement in the wet to consider.
Having done nearly 9000 miles in my 550GT with x-drive, including a 6000 mile trip from Chicago to Florida to Philadelphia and back to Chicago last month - with everything from warm to two feet of snow - my view is that if you have any winter conditions go for the x-drive but if it is for California or such I would stay with RWD.
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2013 650i Gran Coupe xdrive
Prior: 2010 550i GT xdrive | 2008 335i Convertible | 2007 335i Coupe | 2006 130i Hatch | 1998 540i Sedan
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