The three velvia's are very different. You can take a look at these for examples..
Originally Posted by GarageBoy
In short Velvia 100 is known as 'redvia' to many - it pumps a load of red in many situations and kills greens
Velvia 50 is the best by far in my opinion but does go a bit berserk pointed at the sun. It is particularly good at greenery or many sorts and is quite neutral (contrary to popular belief). It moves yellows towards orange and greens toward blue
Velvia 100F is probably the most 'neutral' but hates being overexposed. Skies can go a little weird with Velvia 100F.
Many US photographers like Velvia 100F for desert work I believe - I don't know much about that though..
I'd stick with Velvia 50 for most stuff but it's been discontinued apart from the Japanese market.
You'll probably get on with Velvia 100 but you could try Provia - it's a funny film, good in many situations but goes a bit 'cyany'.