As someone who uses both macs and pcs in the office, I can give you some insights.
Macs went truly 64-bit with Snow Leopard, prior to that, Leopard (OSX 10.5) wasn't truly 64-bit and even some applications like Logic didn't go 64-bit until recently. What irritates me about OSX is that you pay for any point upgrade, even within the same OS version. For example, when OSX Leopard went to Snow Leopard, it was a paid upgrade. Another thing that personally irks me about OSX is that there is no easy way to tweak the OS - you have to learn linux and tweak via the command console... and I've left my C training some years ago so I'm more than just rusty with my Unix.
In any event, 64-bit is only necessary if you need to address lots of ram, and with audio, that's not really necessary, unless you are editing files larger than 2 GB. If you do huge sample libraries, then you need the most ram you can get your hands on. Video editing also benefits from lots of ram. Another thing to consider is the software you will use to edit audio. Aside from Logic and Digital Performer for the mac, you are limited to Bias Peak and some other soft I can't remember right now.
Like I always advise to others... let the software you are going to use direct you on which hardware you will need.