Before trading in the 05 987 base for an 06 S I gave some consideration to the Z4M and posted my detailed review of that car, here:
http://986forum.com/forums/showthread.php?t=5598
I don't have a bias against BMW and currently own one.
Driving characteristics alone, I'd say the best bang for the buck isn't the 987 S, but the 987 base, in that the base gives you 95% of the Boxster driving experience for a good deal less money than the S, and it's pretty clear after you drive them that the 987 is a more enjoyable drive than the Z series.
However, I agree with
Bavarian Motorist and consider the better deal overall, to be the Z4M. Standard kit is
much better and a fully loaded Z4M still won't break 60K. If you optioned a new 987S to match a fully loaded Z4M you'd be spending 70K. The 4yrs/50K free maintenance is also a huge plus, as even a Porsche under warranty is not "cheap" to maintain. When you consider the two maintenance intervals, at least one brake job and the rate that these cars chew through tires you're looking at an amount that most people wouldn't consider trivial.