Is there 100% compatibility running OSX on a PPC? The high price of macbooks always bothers me.
I think the last OSX you can install on PPC is 10.5, Lion.
I voted PC though to be fair, I use Linux. Us Linux folk don't take kindly to being lumped in with the M$ crapware though 
My only regret is the soundtower program won't run under wine. 95% of Windows MIDI sequencers, Patch Editors, hell even MIDI OX runs OK under wine.
It's only when software vendors don't write to specs that badly behaved software refuses to run properly.
I do have a Win 7 install on my machine for cases like these, but dual booting is a PITA. 
Gary
"won't run under wine."
Ok, what is "wine." Not the stuff you drink.
wine is a Windows emulator that runs under Linux when we have to get our hands dirty and run a non native Linux application. 
Generally speaking, if they didn't go too nuts programming in copy protection and used standard documented system calls for hardware device drivers it generally works pretty well.
I've been running Linux only desktops for about 14 years now and have been sequencing my tunes running Cakewalk Home Studio and Cakewalk Music Creator under wine in Linux almost as long.
There are native apps that sequence MIDI under Linux, but for the earlier years they were pretty shabby and crashed a lot.
Nowadays I'll either do most of my sequencing in Cakewalk Home Studio 9.01 or Rosegarden, a Linux app that's come a long way. Cakewalk Music Creator 5.0 also runs pretty well under wine as well, though I prefer the interface of the older Home Studio.
I've also heard that Reaper runs well under Linux as well. I have no idea about Sibelius or Cubase though.
REMOVED
I don't know if it's been asked elsewhere (new to the forum) but I'm wondering about the use of IPad (or other tablets) to complement the computers. I have a Windoze machine (cause I don't only use it for music ) but I use my ipad a lot for musical stuff. I use it to kick samples from time to time on songs and to have others synth. But I also used it, mostly as a test, to have a configurable touchscreen to have added control over the PC361. Used an inexpensive midi interface and I have even more sliders to adjust stuff 
