Drive the 4.8...it's a ton of fun. Since I've had mine, I've driven the M3 maybe twice. I don't think you'll miss your 535i all that much. The handling of the 4.8 is great, it takes corners amazingly well for an SUV, is very quick, and the engine sounds great. I throw it around turns to get on the freeway onramps and it couldn't be more composed. The X6 will be even more fun to drive but the looks are iffy. On top of that, I think the X5 is 100x better looking than the 5 series.

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2017 Nissan GTR Red/black - Tune/midpipe
2013 BMW X5 50i M-sport Alpine white/black - Dinan stage 1
2016 Jaguar F-Type R Gray/red