I'd say Mac. I just bought myself a Macbook Pro 15". I was on Windows and it's not the same world for me. It's way faster and smoother, especially for big projects where windows was slower than my new laptop. But for softwares that only works on Windows, you can always dual boot or download and use a virtual machine.
Concerning Graal, I can play on the client without problems. The RC was made for Mac OS X and I'm leveling with Gonstruct. Everything work perfectly on Mac.