I had a 2009 Cab and now have a 2007 coupe. The '09 PCM was better, no doubt about it... and it sounded better as well. However I remember reading that they stuff more speakers in the cab since it's noisy with the top down.

I spent a pretty penny on a dension unit to get everything my '09 had stock and it's still not as good as my '09.

When it comes to sourcing a 997, there are so many factors that go into it... from color to options to price to year to miles, etc... You're going to have to figure out what's important to you... IMO the '09 to whenever they stopped making the 3.6 is the best years to get. (but obviously didn't stop me from getting an '07)