I would buy the m3 and sell the cayman... lol... i did ;D
I ve been driving a caymanS till the end of 2006. Then i bought a boxsterS ( for the summer of 2007). now i ll sell this and buy the m3. I belive that porsche looses nearly no value by the time ( here in germany), therefor i dont mind "costs", cause there are closely any costs.
I advise you to do a testdrive on a m3 and decide after it.