Neither is going to be anywhere near as good as either 50mm prime, but what I expect would be your best bet is to take your D100 in and do test-shots of a highly-detailed subject (e.g. a telephone book) with the background quite a distance away. Shoot with the 50mm as well as each zoom at 50mm, then shoot the zooms at both ends of the focal range. Shoot several apertures (wide open, f/5.6, f/8, f/11). Check for detail retrieval, sharpness, geometry, and also look at defocused character of the background at wider apertures. Also, try backlighting the subject using a very bright light if possible (e.g. a halogen lamp) to see if there is a problem with chromatic aberration at a high-contrast edge.
I'd say offhand that it's likely that the new 24-85 AFS-G is a better lens than the 28-105, but I would not count on it. I have not shot one, and I was not all that impressed with the previous version (f/2.8-4) as it has distortions at the wide end that I was not willing to work with. The 28-105 is not a top-grade zoom, but it is remarkably consistent across the focal range, which is a good thing.
It's going to be tough to find any zoom that comes close to that little prime. Even the $1500 28-70 AFS is not as good for the most part.
Read my lens page when you have a chance...