I'd say build on your existing Nikon investment. The D100 works with AF-D and AF-G lenses, so if you've got them, you've already cut your total costs.
One a personal note, I consider them both to be very fine cameras.
As I was buying from scratch the main things that swung me to the D100 were -
1) My previous Nikon experience with the CP4500.
2) Looking at much admirable D100 work (particularly Klaus's web site).
3) The fact that a dealer had a D100 that they could demonstrate.
Best of luck in your final choice.