Feature differences are miniscule. My feeling has been that 2006 was the best year for E53 build quality.

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2013 X5M - Sapphire Black
2006 X5 4.4i - Sapphire Black
2002 M3 Euro - Alpine White
2003 M5 - TiAg
2003 M3 - TiAg
2010 535i - Deep Blue Metallic
2007 530i - Deep Green Metallic