I feel the styling on the SLK300 is looking a little dated now. It is a fine looking car, but compared to the BMW Z4 2009 is no longer in the same class for me.
Mercedes were not allowed to offer the same incentives as BMW for buying export cars in the UK. Working in Euros only their pricing was considerably more expensive at the time, this may have been resolved since. Working abroad, this was important.
For your money you get more bang for your buck from the BMW Z4 2009. And it sounds sweeter too!