Tough decision
You're having a hard time making the decision because the alternatives are so similar.
The G5 1.6 has a fast (but not the fastest) single processor.
The Dual G4 1.42 has slower processors, but they're still pretty fast, and you get two of them. Put them both together, and you have a slightly faster machine for any task which will use both processors.
See why it's so hard? And why they're priced so similarly? And why nobody can really give you that one fact to help you make the decision? Because they're basically the same.
Since they're so similar, you might as well just take the newer model, the G5. It'll probably have higher resale value down the road because in three years, everyone will have G5s and G4s will be like G3s now. (Even though a high end G4 will still have acceptable performance.)