I'd keep the M5 unless you don't mind taking a serious bath on depreciation. Only reason to swap would be to get back in a manual, a very valid reason admittedly. Both cars are heavy so I don't think you will notice much difference there. M3 is smaller but still a very big car to hustle down a backroad. No one has driven the M3 yet though, best thing to do would be drive a demonstrator when they hit dealers in March and your gut should tell you if it makes sense or not.
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