Both systems can do music production well and frankly if you are paying attention to your software/hardware needs they are identical.  It comes down to the philosophy of the machine.  MACs historically come with more bells and whistles and are integrated very well, but are tougher to modify later on. (i.e. swapping in new hardware etc.)  This would sometimes sticker shock people over to PCs which (Desktops) are very easy to modify with quite a few different options in hardware and software so are more easily customized over time to a specific setup/application.

If this is just a stand alone recording machine just compare the hardware/software end of things and this might end up pushing you in one direction or another.  If you want to use the machine in a gaming or office environment the PC will probably have more options.