I had the same dilemma when I was trying to decide which car to get. Always loved Bimmers but the Cayman S really intrigued me and there are far fewer of them on the road so I made my choice and went for the CS. M3's are great cars in their own right so either way you'd have a great car. Very Personal choice and you won't run into folks here that will give you (much) of a hard time for choosing something else.
I knew I'd want to 'uncork/delimit' my CS as soon as I got it so I planned on immediately making some easy changes to get that 5o HP bump right off the bat...
I picked mine up and immediately drove across country to beak it in a bit and then dropped it off with Imagine Auto to have some upgrades done and do my best to reverse some of the limits that Porsche put on the CS to try and protect the 911. New full exhaust, performance filter kit, ECU tuning, limited slip dif... I got my 50HP and improved the sound too. The difference IS noticeable and I LOVE it! Yes, I'd really like to have another 100 on top of that and there are options coming up that seem able to deliver that. YES, its way more money that a new v8 M3 stock and even more so once you start making changes but its a passion. Follow YOUR passion and few will fault you for it...