The Velvia 100 (not 100F) is actually quite similar to the 50, in my opnion. Technically it has lower grain (RMS value), though sharpness doesn't seem to be quite what the 50 was (which was famous for that). Color response is similar, saturation is close. If you underexpose slightly, you'll increase saturation a bit. So perhaps Velvia 100 @ 125 might look almost the same as Velvia 50.
I think Velvia 100 is a tad more yellow, too, but just a bit. This is based on about 9 rolls of the 100 after 20+ of the 50.
allan