No. You pretty much answered your own question. If you were going to buy it outright, you'd be better off with the M. But if you are going to lease, the 50i will give you what you are looking for at a substancial savings. It's not like the 50i is a slouch in the horsepower department anyway !

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2013 X5 Xdrive35i Premium
2013 F30 328i
2004 Mustang GT Very Modded...