Just depends on what one wants the image to look like. Long exposures tend to match what we see...persistance of vision and all that -- we tend to take in the launch, peak and decent of a firework as a whole, when in reality, the tail of the firework can already be disappearing as the firework peaks.
Also it is easier -- less worries about timing the shot.
Ah, right, but I am not so sure I can use it here, I dont have a high vantage point close by, I'll try anyhow!
BTW, any other care I should use with Velvia? Ill be shooting that in 120, and using a d/film SLR