I usually use Tiger in machines for "specific" things (OS 9 Classic, Speed, hotless) , it's cool if you have more than one OS in the same machine.
When I got my first PPC Mac (12" PB 867Mhz) I installed tiger only one time and, 3 years later of intensive gaming and use (tigerbrew,ageII multiplayer, pcsx).
Based on this, I'll ask:
1) is your only PPC Mac?
yes: install Leopard to get another few goodies.
no: Install Tiger