Meh.. I've driven an ~09 cayman up in the mountains a good bit, I own an 08 z4 m, and have put about 300 miles on a 11 z4 35is sportdrive or whatever the top one is supposed to be..
The 06-08 z4 drives more like a cayman/boxer than the new z4 but it is rougher. Shifting, steering, everything feels a bit thicker and solid. It feels like a hooligan machine. You have to think about corners and how to approach them, deal with the rear end sliding out while having a huge grin on your face.. it's a beast of a little car. Personally I think the suspension is a bit tight compared to the others, but that's "part of it". I would NOT get any of these as an only-car daily driver anyway.
The cayman is a precision tool. Everything fluidly slides into place and moves with little resistance. The interior is spartan and boring, imo, and I'm a porsche fan (had 914s, ghias, etc). It's a GREAT car, but I wouldn't get one unless planning on doing track days. It's a precision tool, and reminds me of a razor sharp scalpel. You just point the front where you want to go and give it gas and... it goes. There's no screeching, no tires breaking loose, it feels like one of those toy cars on rails. It's precise, but it's not quite as fun as coming in with a huge chainsaw with dust and dirt billowing everywhere as you slide through a corner, imo anyway.
The 09 and up z4s are a joke. They're floaty, big, bloated. They feel extremely heavy even though they don't weigh much. The interior is laid out well, but very..... feminine. They rattle, the tires/suspension make funny noises when pushed hard, about the only good thing is the motor. It is defintiely in a different class compared to a boxster/cayman/370, in my opinion. I would not even consider buying one to be used for any real 'sports car' purposes.
...all of this said, I recommend you actually drive some of these cars. You saying the >09 is a better sports car makes it pretty clear that you're doing a lot of magazine and reading comparison stuff, and all of these (350/370/z4/boxster/vette) drive VERY different from each other even though their track times are very close. I have a buddy who bought a TT based on looks and magazine reviews and has now decided he hates the driving feel. Whether you use carmax, post up on boards, car dealers, I would find a way to test drive these and compare.
Also, I've posted about this before, but I wouldn't discount the idea of flying somewhere to pick up a car. When buying something like this you're much better off spending an extra $k or three and getting something with a solid service history, one owner, taken-care-of car. Avoid dealerships, especially ones that claim they know the previous owner who was an old couple.