I would stay away from the C4 and go for the C2 less drivetrain issues then the AWD
I am in the same boat as you with my next 911, not doing 993 though.
Does the AC really work better in the 964?
You can always upgrade the AC with more modern parts they have now.
I personally think the 964 adds more bang for the buck when it comes to upgrading from the Carrera 87-89 models.
But the 87-89 are more truer to the 911 feel...
It's all what makes you happy in the end