I think this can be summed up quickly: White tea is healthier than green tea -- it's more potent. In effect, the more processing the tea leaves encounter, the less potent they become: black tea (most processed) < green tea < white tea (least processed). All three of these tea come from the exact same plant/leaf.
This may be true, from an EGCG standpoint, on a leaf comparison basis, but one more thing to add, drinking a powdered green tea will be better than a water extracted white tea. This is because a lot of the EGCG is not extracted when the leaf is put into hot water, although most the caffeine is. I have not seen the numbers on white powdered tea, but it should be the best, then powdered sencha, then matcha.
I drink powdered sencha as sencha was what the japanese green tea research was based on, it is definitely powerful enough to easily get the studied dosage, and I prefer the taste over the white tea powder that I have had.