I was in the market for a 3rd car as a fun car. Initially my wife and I looked at an '06 MX-5, but the deal fell through because the car was a nightmare. I got frustrated and upped the ante to the S2000. I fell in love with the idea of the car, but when it came to driving one, I was frankly a little "depressed" at the value ratio. Don't get me wrong, its a good car, but a little too go-cart-ish, especially for the money they command up here. To add to the trouble, I couldn't find an S2000 owner that wanted to part with theirs (I even offered a guy over his asking price and he rejected). Frustrated with that venture, I came across the M Roadster for about the same money.
I found that the Ms were generally in better condition (lower milage) here, and wow, apples and oranges in terms of power. Yes, the Z4M is heavier and bulkier, but for all the complaints that I read in reviews that it is stiff and harsh, it is nowhere near the S2000. We love the M, and while I realize the maintenance and general ownership costs will be higher, in terms of a street car, I think it wins hands down. The interior is such a better place to be, and all the creature comforts (memory seats, fully automatic top, better stereo, awesome steering wheel) make this a comfortable daily driver.
I'm sure you'll love the S2000, its a good car, and on the track it may even take the Z4M, but as an "overall", I simply love the torque of the M engine....puts a smile on my face every day (and hey; it revs to pretty much the same as an AP2 S2K)