Get any AMG if sound and tire smoking is most important to you.
Get a Z4 if you want an open 2-seater which both looks and feels great to drive no matter where.
Get an SLS AMG roadster if you want both and only have few days left to live...
And yes yes, get an M3 if you want a luxury track / racecar and think the 911s/Boxters aren't good looking. I personally do not regard any regular AMGs/Audi RS/Z4 as much as racecars as an M3. Probably the AMG black-series is? But again, I have been involved in racing most of my life, and its not all that important. Imo, if you like competing on a race track, go-karting is all you need.
Over steer is fun. High cornering speed, control and stability isn't really. Its disappointing if you used to have it, and then lost it, but so is a higher top speed - and both top speed and cornering speed is not really useful for much else than bragging. OK, alot of people will disagree, and we can't all be the same, but I still think over steer is more fun. I like a tossable car, even if it is slower around a track.
Now, some people might tell you the Z4 understeers, and the C63 AMG is better for drifting. In my experience, as a Z4 owner, IF I loose traction in a corner, 9 out of 10 times it is because I am provoking the rear wheels to play. In fact, I can get the tail out in almost any corner I want. I am sure an AMG could do it with more violence, but then personally I never had this addiction to tire smoking that some have. And in those 1 out of 10 times where I do get understeer, I am simply going faster than expected, or not aware of how slippery the surface is. This car hides its speed well, and I actually need this safety feature so that I can slowdown without spinning!
So it comes down to roadster vs coupe. Looks/attraction vs great sound. Sports/comfort/economy balance vs sheer hooliganism. You are looking at two very different options, and if you cannot make up your mind, well, there is always that SLS roadster haha...
Little known facts about BMW: 1) The N54 is still in production and used in the current Z4 35i and 35is. 2) In a Z4 you can get sport+ steering combined with normal suspension etc by selecting DSC=off. (Update: DSC=off also offers sports steering in the M135i) 3) All Z4s with 19" wheels are still only offered with the worst performing OEM tires ever sold with a DCT equipped BMW (Potenza RE 050 RFT).