To my tastes, Astia is the best all-around transparency film by far, but I do also find that Velvia is sometimes the better choice. Both are essentially grain-free, though they are rather different emulsions.
If you happen to have a lens or scene that is especially contrasty, Astia may be required. As already mentioned Astia also has a more accurate response with reds and skin tones. It also pushes to ISO 200 with almost no penalty.
In lower contrast situations (say three stops of dynamic range) Velvia adds snap and saturation. It also gives the appearance of greater sharpness due to an enhanced local contrast/ adjacency effect-- much like Kodachrome.