They say that the only worry people have right now is Apple abandoning the G3 chips which also wouldn't be good for "business" since there are still a lot of G3 users out there (like schools, like myself, etc.).
Though by observation, again, Tiger runs a little slow on G3s right now unless you boost it with more RAM, and even at that (like I did with my iMac G3, had 1 GB of RAM in there), it doesn't help much.
I would imagine--pure speculation--that Leopard would crawl on a G3.
I digress.
I bought this G4 I'm typing on right now knowing that, well, confident that it would last me for at least 5 years, save natural disaster or something.
You threw in a good factor there. Of course, the MacBook would be "compatible" with any new OS that rolls out for a good time out.
However, even at that, with the way things go with technology these days and how fast they update things, it will always come into play.
Like the MacBooks. They've updated them three times within a year of their existence! The 12-inch PowerBooks? Took about 1-2 years before they revised it (there were four speeds) a second time.
Anyway, I would suggest you also do a Google search for your question "PowerBook G4 vs. MacBook Pro" and see what you get.
YIKES! I just realized your initial question was MacBook PRO, and not the consumer-oriented MacBook. That would be another thing to think about, since in a way, the PowerBook G4 is the MacBook Pro in terms of feature sets. The MacBook is technically the iBook. Though it seems that the consumer MacBook would surpass the professional PowerBook G4 since it's newer, which I beg to differ personally, you can look at it that way too.
(Remember how I turned down the MacBook Pro because of price and went for the PowerBook G4. I never would have even considered the iBook G4, why I also didn't want a MacBook. It was just like you, for me. PowerBook G4 or MacBook Pro??).
But again, it comes down to price for you, as you pointed out. You get what you can afford, and then...? You get what you pay for.
Just to throw one more thing in, take Final Cut Studio. It will not run on a MacBook (though I'm sure someone will find a hack) because it doesn't yet have a dedicated video graphics card. My little G4 PowerBook? It's on here! Ha ha ha ha ha. =)