in terms of lower initial cost, lower subsequent cost, lower depreciation, lower maintenance cost, lower road tax cost, lower fuel cost etc, m3.
for everything else, m5.
is cost a bigger incentive than emotion for you? are you the kind that mimick the sound of revving engines when you run up the stairs at home as a kid? are you the kind that particularly enjoy washing your car yourself for the reward afterwards-the chance to drive 10/10s after a wash to dry her out nicely? the type that'd cook and eat in so you can save for the brembo monoblocs 10 months later?
if answer is all yes, that's a necessary but insufficient condition to go for the m5.
the final hurdle where most falter then, comes back to cost.