I've shot Antelope Canyon with Provia; it is not quite as punchy as the Velvias but has slightly better exposure latitude. I would avoid Velvia 50 for slot canyons due to its relatively poor reciprocity characteristics (any of the newer films such as Provia, Velvia 100 or Velvia 100F would be much better in this regard). Actually Kodak E100VS is one of my favorite red rock country films, as it does a fine job in recording the nuances of reds and oranges found in the area, but I don't use it for slot canyons since its reciprocity characteristics aren't as good as the films mentioned above.
Frankly, when I shoot Antelope again I'll be taking along some color neg film (Pro 160S or Portra 160VC), since scene contrast is so high it's often difficult or impossible to hold highlights while retaining shadow detail.
For shooting cottonwoods during fall colors, I would suggest either E100VS or Velvia 100, as both do an excellent job of recording vivid yellows. I would probably favor Velvia 100 due to its superior ability to push (to freeze wind motion among the cottonwood leaves).
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