Okay, I'll take some time for this.
This is coming from a user of BOTH, yes BOTH is possible
I just recently traded up my Powerbook 1.33 for a new Dell 6000D tweaked out.
Well, the Powerbook isn't bad at all, and I really can't say too much negative about it. I loaded it up with 1.25 GB of ram, so it ran right, but, GET A MOUSE for it, you'll find yourself very frustrated using the touchpad (it kind of sucks). Once you have the mouse though, it becomes a better experience.
Hardware wise. Looks on the Powerbook are GREAT, I liked how it has been designed, and you can tell that there IS great engineering in the build quality. I can't say its the same for the actual hardware quality. My LCD screen had to be replaced 2 weeks into it (backlight thingy). They did have GREAT turn-around and had it fixed in a jiffy. Screen Size = the suck
The OS. I love the OS, just had to get used to parts of it, I love parts of it, meaning, when you need it to work, it works, no closing out of crazy windows, etc., its just clean running, thats the best way I could put it. A negative with the OS, multitasking with it is not as easy as windows and can become frustrating in how the dock is setup, how the windows are NOT easy to size up, or get maximized to full potential, and there are other little things that I just generally dislike about it.
Now, for the PC purpose of things.
You will be getting much more out of workspace on screen, you'll be able to much more depending on what screen you do get (I got 1900x1200) but most people opt for the one lesser than mine.
You'll be able to have that memory bumped up nicely, as well as having a great running video card and it will be speedy throughout.
Another thing to consider midi wise is, you have the options of the PCMIA cards that are basically upgrades to the integrated sound and can provide different midi hookups which I'm sure you'll need. You'll find a much bigger niche market of sound/midi products for PC, even though most people say Apple is king here, why? Because more pople use PC, its just a common fact of life. I would not suggest using the trackpad either, it just kills the useability of the notebook.
upgradability that does pretty much kick apples butt (processor, video card, etc., sound) When all you really can upgrade for performance on the mac is = the HARD Drive.
Support will be about even for both of them, unless you are the type to call in for stupid software problems. HARDWARE support (replacement of parts) will come in about the same time.
So, hopefully this is a non-biased post about how I feel on the differences.