There should be a few threads about the differences between the Velvias (including the now-discontinued-but-still-available-in-places Velvia 100F) in the archives. In short, I greatly prefer Velvia 50, which has essentially the same characteristics as the original Velvia introduced in 1990 and discontinued in 2005. Velvia 100 has higher contrast and a different palette that gives you bluer greens, redder reds, and weaker yellows. It has its appeal, but I got sick of it quickly (mainly because of the contrast - it loses detail in highlights and shadows quicker than 50) and now use 100 mainly for long night exposures (much better reciprocity characteristics than 50) and situations where I really need more speed and won't mind the colors. Of course you should do your own comparisons, but as far as my preference: on a recent trip to the southwestern USA I used 39 rolls of V50 and 1 of V100 - and that one mainly because it was about to expire!