I personally hate the look of the RR but when I bought mine my M my close friend bought a RR Autobiography the interior blows away the M but the M has more of an edge to it. It feels more sports car like as well and the performance isn't even close even though the autobiography has a pretty good bump in power of the regular SC. I am late 30s and the M makes me feel my age the RR made me feel about 55 when I drove it. I also like the fact the M has a lot more aftermarket support.

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X5M Eisenmann exhaust, de-Catted down pipes, 22" HRE wheels, ECUProgram stage II (700hp) tune
X5 35d Tuned and Lowered