have you tracked your Cayman S?
get a E90 M3 to replace the 335 in 18 months, unless you push your cars on the street and track whats the point of two high end 3 series? from the drivers seat they look nearly identical, whereas the Cayman is a very different driving experience, 600 pounds less, tossable, very happy at the track nipping at 997s, vettes and more powerful cars.
unless you just want the newest M car...then just get the M3, but I think you'd be shorting yourself.
I bought my CS as a temporary car until I get an M3, now I'm planning to keep it and sell my other 3 series family sedan, I'm going with a sedan, 414HP 4-door daddy car, and use the CS as a weekend, date, errand, track car.
calling the M3 cheaper doesn't make sense, your going to take a bath on a 07 CS compared to buying a new M3. An average 2007 CS is worth about $50K, you'd need to make up $10-20K all in.