Boy. If you are new to G3 and G4, it would take quite a bit to explain the differences.
Among the two options, the G4 will be faster, not only because the chip is better, but also due to the fact that the G4 machines usually also have better system bus speed, better graphic card, better ram, etc.
Also, there are a lot of software out there that takes specific advantage of G4's strengths such as Photoshop.
My suggestion to you is to get a new iBook G4 or the PowerBook. You will be happier in the long run. If you are low on cash, go for the G3 iBook as it is still very functional and practical. I am sure you can find a good deal for G3 iBooks, new or used.
Cheers.