I use both, mostly the Classic 50 (also at ISO 40) which is warmer and renders stronger colours. The Velvia 100F is good when pushed to ISO 200 as this warms it up and gives a useful speed increase when shooting in the forest in wind etc.This is how I now use this film.
The 100F does not pick up pink dawn skies at all well, alarmingly so! Which you can see in the film test on my website www.baxterbradford.com in the "Just in" gallery. The film test is on the "Tried and Emulsional" link at the bottom of LHS frame, below the red anorak alert warning!! Sorry I couldn't find a direct way of supplying the link to you. That's frames for you I suppose.
I did this test to see what to use in different circumstances. Sheet film made it easy to switch near instantaneously using same filtration.
Just to complicate matters, I understand that in Japan there is Velvia 100 (not 100F) in Japan which is much more like the Velvia 50! This hasn't officially reached the UK yet.

Bax