I own an SLS but not an M5, mines an F12 M6 Cab. Compared to the SLS I do think their a little similar but after about 70 I think the SLS will pull away. What makes the SLS amazing is high speeds feel like nothing even compared to my F12. There are videos of SLS's pulling away from 458s. Granted the M5/6 are half the price, much more comfortable and have either 2/3 more seats they are in my eyes an even bigger accomplishment.
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