I have owned the Z4 Manual and its a great car, I would stay away from the Crossfire personally, the only good thing about the Crossfire is the Mercedes engine inside, other than that the workmanship and reliability is far superior, my Wife has a Lexus SC430 which is less aggressive and a lot more car for the money. my 2 cents, happy shopping!
since I was in a bad accident in 2009 I want something large enough to protect me when I am driving. Since the Crossfire is small & so is the BMW I would steer clear away from them . Get something that will offer you some protection if you are in an accident....JMO...
With all of the debris on the roads here in the Houston area, I would be terrified to ride in a convertible or on a motorcycle. I drove a convertible when I was in my 20's and once the novelty of driving it wears off, reality kicks in and you long for something with more room, better protection from the harsh heat and sun and something that is more visible to the huge trucks and suv's here. I cant tell you how many times I've almost smooshed a tiny sports car when trying to back out of a parking space. If not for the beeping of my back-up sensors, I would have never seen the car.
Get the bmw. They are really good cars. We have a bmw and love it, they call them the ultimate driving machine for a reason. There so fun to drive . Bmw is a way better madr car than the other one. Have fun