The S2 isn't a bad box, in fact the sensor in it has some interesting capabilities especially how it works.
You see, it pulls information in at 12 megapixel and downsamples it to 6 megapixel thus producing what you could claim is a better and sharper image.
However, you're also looking at cameras from a different market.
The S2 Pro was originally designed to compete against the D100, not the D70.
The downsides of the Fuji S2 Pro fall down usually around the physical side of things and some of its recording capabilities.
For instance, it's lack of any real battery system like most other cameras have is a concern, and while the batteries do tend to last a while, you're still left with having to load several batteries into a box as opposed to just the one battery pack, as far as I recall.
Also, if you're used to the way a Nikon feels... this camera feels different... and in my opinion, not a good different. The grip on the right side feels like it's got an awkward width on it and as a result doesn't feel good in my hands. But that's a personal thing.
I haven't seen a comparison between it and the D70, but the D100 and the D70 are fairly similar, so you might want to start
here.
I know at least one of our members, I
think it might be Spada, has a S2 Pro.