There's a number of reasons why I took the dive, the biggest ones;
a) The deal was a no-brainer... if ever I wanted an '06 40k mile Carrera, I would usually have to spend at least £4k more than I bought this for to get my hands on one at all.
b) Granted I lose the soft top... but the novelty has worn off for now, I may come back to driving a convertible at some point in the not-too-distant future, but for now, the novelty of being 24 and owning a Porsche 911 is far more appealing, and a 997 at that! (would not have considered the 996)
c) only having 2 seats was becoming an inconvenience, and it would become harder to live with now that my gf is living with me and I am often ferrying people about
...on to the fun stuff...
The BHP is marginally less than the ///M, but the Carrera has more torque from the 3.6 engine, the overall stats are impossible to draw a line between. The massive wave of said torque you get once you are moving is just incredible and the way it enters corners is so much sharper and balanced, I have already been able to drive it a lot harder than I would drive the ///M, the ride is far more comfortable, and I get a little more ground clearance.
Lastly, I actually have connections at Porsche so I can start getting better deals on parts and servicing (which I didn't at BMW).
There is no denying that the ///M is an incredible machine, but there is a big gap between it and the Carrera, while the Z4 is a very fast and powerful sports car, people keep telling me the Carrera is closer to a supercar.... and I rather like that!