I had a 2012 x6m and now have a 2014 50i. The 50i is much nicer to drive it's more comfortable, way better fuel mileage(if that matters). The x6M felt a lot more aggressive though I had it tuned with Velos and the 50i with juice box.
I can't say which is faster but I far prefer the x5 50i, just wish it was louder
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