Windows. Made by M$. Uses the ntoskrnl.
The main advantage to Windows is all of the software, and a huge user base. Most major distributors use Windows as the default OS, therefor, there is very good support out there.
Main disadvantages: You are going to NEED that support.
Windows uses dynamic link libraries to run it's guts, so if one of those catches a cold, your whole computer gets sick.
Mac OSX = Unix = most versitile OS ever created = 40 year track record.
OSX is very user friendly. It is also very pretty, and frankly uses a bit too much juice to make the pretty. Don;t get me wrong. I LOVE my OSX leopard, but it is a bit top heavy.
If you are going to be doing a lot of graphics, or music work, Your choice has really been made for you... get the Mac.
Linux = Increasingly competitive with the other desktop environments out there = 100% customisable = open source = you actually get to understand what the machine is doing, down to the last bit.
Sooo..
I have a lot of work to do = Windows
I'm creative and want a machine that accents that = Mac
I want to understand this stuff = Linux
I want to thumb my nose at the establishment = OpenSuse 11 (A linux distro that works like Windows, and looks like Leopard.)
Now....
Here's the big deal breaker.
Get a Mac, then:
Partition the drive in to 4 parts.
1 for OSx
1 for Windows XP (Don;t do Vista yet if you can avoid it)
1 for your Linux.
1 for keeping stuff like documets and files to be read by all of the OSs
You don;t really have to pick one... you can heve 'em all.
(PS You can do Windows / Linux on any windows box, but if you want OSx to work the way it is supposed to, you'll need a actual Macintosh.)