Have you had a chance to test drive both the 50i and 35d? Best to experience in person, but I don't think you'll be disappointed paying a bit more for the 50i M-sport. Great exhaust note (different from standard 50i) and plenty of power.

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AtlX
2014 X5 35i M-Sport While/Black (wife)
2013 X5 50i M-Sport Silver/Black
2011 X5 50i Silver/Black (retired)
2008 335i Convertible Space Gray/Gray (retired)
2004 M3 Convertible Blue/Gray (retired)
2001 X5 4.4i White/Black (retired)