There was an interesting article in Rifle's Handloader magazine an issue or two ago, where the author showed that volume measure was a better way to get the same results when going from lot to lot of a specific powder. Because powders are not 100% consistant between lots, weighed measures will tend to shoot differently. Whereas volume measures, between lots, will have less differences than weighed measures.
I haven't been reloading long enough to really know... So I take it with a grain of salt and I try both. So far, the author has been spot on.