I found some information for what it may be worth. It appears that approximately 2000 SLKs were delivered in 2010 v. approximately 3800 Z4s. I don't know for sure whether these are calendar or model year numbers. If true, (1) Neither line is setting the woods affire, and (2) I would expect the numbers to more or less reverse with the introduction of the R172 car line. Lots of people do like to be first on the block!
I became curious because I believe there are at least 5 or 6 Z4s to every SLK in Central Texas.