Have you driven both?
I know the Cayman is in another class, but which one would you prefer (including its costs) for your intended purpose?
We picked up an old Miata for duties that we want to retire our M from. And damn, I am having so much fun for so little $. Part of the appeal is how little we spent for it. We're still kinda in shock actually at the smiles per miles over cost ratio...
2013 M3 coupe (Azurite Black, Black, Piano Black)
+ miata nut (2 miatas) + Mk6 GTI 5-door hatch (our mini-SUV