The number one reason not to get a PowerPC-based machine right now is that the architecture has been a second class citizen at Apple since the Intel processors were released. There are more bugs in Leopard for the PPC machines for instance. Optimizing is probably no longer being done the same way it is for the Intel architecture either. When you see people reporting about speed increases in the 10.5 Leopard OS versus 10.4 Tiger, they're talking about Intel machines. On a PPC it's either the same or slower.

10.5 Leopard is also likely the last OS we will ever see for the PPC machines. Apple hasn't officially announced this, but I have it on good authority this is the end of the line. 10.6 was designated Intel-only and that isn't likely to change (could happen but I won't bet on it).