Best performance for the price is the PC, hands down. The Core-i7 3930K should be the sweetspot for price/performance right now. It will enable you to build an extremely capable system with 32GB ram (excluding drives) for around 1000 eur. With a couple of SSD's added I don't know if you would ever run into the CPU limit.
Windows 7 is also better performing than OSX for audio applications.