I currently have both.
In favor of M3:
Last of the 8 cyl, NA M cars, get em while you can
Pulls harder than the Cayman
Great sound of the engine
More tech features standard or less money than P.
Love the ability to stream music via bluetooth, pain in the butt having to plug my phone in in the Cayman
More space for passengers and luggage.
No maintenance costs for 4 years (except for tires)
Much less expensive than the Cayman unless you're really stingy with options
In favor of Cayman S
Better overall fit and finish
More exotic looking, draws more attention (can be a + and a -)
Better club events like DE, back country drives and Autox
Better transmission for track, PDK in sport+ holds every gear till redline
Better fuel economy (not even close), especially regarding range, M3 wouldn't be as bad if it had a bigger fuel tank, seems like I'm always filling it up.
It's a Porsche!