First thing you should do is enjoy the 328i while you have it. I just recently traded in my 328xi for my Z4 and I have to admit that I am really missing my 328 right now. It isn't that the Z4 is a bad car in any way, but it is entirely different than the 328. As for your question, I guess only you could really make that determination. The most important thing to me when I begin looking for a new car is does it appeal to me aesthetically. If I don't find a car attractive then there is not much of a reason to go much further. That doesn't mean that the look of a car is the most important factor, but is the first criteria. Keeping that in mind, I tried to like the Porsche Cayman and even looked at them a few different times but I just couldn't like it enough. As for the SLK, I kind of see that as a snotty country club woman's car. Obviously that is just my perception, much like I look at a Volkswagen Beetle as a girls car but that is what would keep me out of the SLK. Now, both may be fine cars, but between your choices I would definitely lean toward the Z4.
BMW 335is, Le Mans blue with Black Dakota leather, Convenience and Premium packages, 19" wheels, 7 speed DCT, heated seats, Nav., Harmon Kardon Logic 7 stereo w/ Satellite radio.