I would go for the mac for sure. I changed over to a DAW in 2005, and got myself a 2.7 ghz dual core Powermac with Logic 7.3 at the time. It was a huge learning curve seeing as I was used to hardware set-ups before getting a mac. I got a uni-body, all singing, all dancing macbook pro last year with Logic 9, and I totally love it. I'm using a G-Tech external FW 800 drive, and an Apogee Ensemble interface. I got the UAD 2 Solo laptop card to run some of those tasty Universal Audio plug-ins (Neve eq's etc), and I just got the UAD Satellite which is daisy chained via FW 800 to my external drive, then my interface. I can still use the Solo laptop pci-e card, so I can now run three time the amount of Universal Audio plug-ins than I could with just the Solo laptop card. My whole set-up is sweet as a nut !!!
Logic 9 comes with some decent virtual instruments, especially the rhodes and hammond etc. I also use Arturia soft synths, and have the Jupiter 8V, Prophet V, and the Minimoog, all great sounding synths. If you scour ebay you may find someone still selling Arturia synths with the voucher that allows you to download 2 for the price of 1. Also for drums, check out Xln Audio's Addictive Drums !!! All I have to say is mac, mac, mac, all the way. You wont regret it !!!!