[I installed Debian, but I'm thinking of switching to Xubuntu.
Which is best for my 600mhz 256mb ram powerbook/ibook powerpc?]
600mhz speed is okay, but be aware that 256mb ram could possibly pose some issues at installation that requires you to enter parameters at grub loading. I have tried a 512mb ram hosted on an ATI with 1.5Gz of processor, kernel panics under Ubuntu 8.10 & 7.04, other distros that failed were the latest releases of Suse, Fedora & oSolaris. But when I tried Ubuntu 6.06 installation went smooth: so that means 'backward compatibility' often solves the problem of aging pc's.
As to your question: "which is best?" it depends, but i make my bet with Debian considering the latter's wider support on various architectures, it should make sense when our notebook is quite older than today's.
Here is a proven criterion to avoid headache: Try get the year when your hardware was released by the manufacturer; then get an Ubuntu or Debian release somewhere that or near that year. I assure you, with same ages they understand each other. Although, security-wise, we have to admit some concessions.
Hope it helps.
[Reminder: Linux & Unix users pound keyboards both to control system and often in applications; it is wise to obtain an extra keypad if you are heavily using the laptop.]