Go with a pc and buy a good video card that can take advantage of Mercury graphics engine.
source:
http://www.pugetsystems.com/labs/art...celeration-161
You can look on Nvidia about Cuda it really impressive how much time you save
http://www.nvidia.fr/object/quadro-p...op-cs6-fr.html
the price for cuda enable video card start arround 200$ so it not that much. It work with video encoding to, mine have go from 4 hours to 20 minutes.
Also you should think about getting a SSD it one of the adobe recommendation to improve photoshop (if you already have maxed the ram)
http://helpx.adobe.com/photoshop/kb/...id_state_disks