Mainly, it's two stops faster.
That makes a HUGE difference, a lot of the time. I found myself at an outdoor outing trying to shoot photos in a shaded picnic area and thought I had brought Provia 400X but only had E100G. 1/8th to 1/15th wide open - not good handheld. I didn't bring flash either since I planned to be outdoors with 400 film.
It's also rather lower contrast and saturation and often, to me anyway, looks more natural than Provia 100F. But if you like Velvia and it suits your subjects and style, you'd probably like 100F less and 400X even less.