Refinishing 100 yr old fir floors that will see a fair amount of traffic is certain areas. Which finish is more durable, least likely to show wear, scratches, dirt over time?

We normally use a satin finish, it tends to show less wear and tear. According to the manufacture there are more solids in the satin than semi or gloss. Not sure if that makes much difference in wear or not.

Gloss is the hardest from what I understand. The satin/semi additives cause a softer finish. Isn't that why moisture cure is gloss?

Well, up here, or at least in Quebec, you can get MC in Satin or Semi as well as Gloss.
The old school (me, lol) used to always put gloss on for all coats but the last one, so, usually two coats of gloss top-coated by one coat of the desired sheen level. The reasoning was two-fold. First, less chance of streaking and other problems associated with the satins, and secondly, gloss is slightly harder. The reason I say slightly is that all polys want to be gloss, but we cleverly add silicate powder to the soup to deflect the light hitting and rebounding in such a fashion as to reduce the gloss. This only affects hardness to a small degree.

I did some checking with the manufacture and they informed me that there is no difference in hardness, the hardness comes from the resins and is not affected by the dulling agents.