Thanks for all of the advice so far.
I don't have anything at the moment, but I'll count your post as a firm endorsement of Windows.
I mostly use a computer for web browsing, word processing, lightweight image editing, email / IM / Skype, and programming. Linux suits me just fine for all of those things, and I'm spoiled on how easily I can find and install new software with Debian's package management.
It looks like you're recommending against Linux, since all of the programs you mentioned are either Windows exclusive (Sonar, FL Studio), OSX exclusive (Logic), or Windows / OSX (Ableton, Cubase). I'm not sure, though, why you say the choice of operating system isn't a starting point, since it's the first piece of software I'm going to put on the computer.
My main goal is to do what's going to be as easy as possible, so Windows sounds like my best option based on what I've heard so far. I don't mind putting effort into something, but I'd prefer that the effort went into my actual music instead of things like just making my setup work in the first place.