OK, so I'm going to weigh in here. While I'm not as technically advanced as some of the others, I do have about five years of experience with several forms of Linux including using the Ppc version on two G4 Macs.
The simple answer to your question is Lubuntu 12.04, and I say this for a reason. I found both my Macs to be grumpy and particular about what OS they would boot. I can't tell you how many versions (mostly of the 'Buntu family) I tried that simply wouldn't boot on these machines. Lubuntu 13.04 is one of them. But 12.04 has always booted on both. It is still being supported (even though the timeline of "Buntu life cycles tells you it shouldn't be), and when it's not you can simply upgrade through the update manager saving all your settings.
I don't recommend running an older version as you will not get any security updates and most importantly, you won't get any browser updates. On some websites, you will simply be told your browser is too old and you won't be able to open the page. This circular firing squad of "the old equipment no longer is supported by the new software and the new software won't run on the older equipment" is exactly why I left Mac OS behind for Linux.
I tried Debian and Mint Ppc. I liked both, with a nod going to Mint as it ran faster - it might have been the snappiest one I tried on the Graphite "Sawtooth". But with both, the browser doesn't automatically get updated when you download all your other updates (unless it's changed since I fooled with it). You may be OK with it if you don't mind backporting your updates for your browser, but that was all a ittle too geeky for me. The support from the help forum in Mint is next to non-existant, and I felt that the Debian people, while in much more abundance, have little patience with a raw newbie. With Lubuntu, this stuff is all point and click, and in my opinion, the support community is a little more forebearing.
I didn't see anyone mention it here, but you must know by now that Adobe Flashplayer no longer supports the Ppc architecture, meaning no direct playing of Flash videos - unless you want to run OS 10.4 Tiger and Use the Tenfourfox browser, but then you're back to using obsolete software with no security updates. Gnash (available in the repository) will play Youtube videos in a hurky-jerky fashion on my emac (1Ghz. processor), but crashes on the old Sawtooth (450Ghz. processor). However, using Video Download Helper plugin in Firefox yeilds near HD quality video play on either machine...you just have to download first.
Having said all that, I don't in any way regret my move to Linux. It may not be as "polished" as Mac OS or Windows, but the differences are small and insignificant to me. And most importantly, I get to decide when my equipment is outdated, not the manufacturer!
