Black worm is not a true culture. I've never heard of an aquarist propagating them. Have you?
"Better" can mean different things to different people, e.g. cost, ease of keeping/propagating, availability, nutrition, etc. I've never used White worm, have always heard negatives on them. Just last week I met up with an experienced breeder of different wild Betta species who had experience with them and swore he'd never use them again.
Years ago when I was trying to figure out which foods were most preferred I'd feed one type of food to satiation, then offer another type to see if it would be accepted. Try feeding frozen blood worm, for example, until the fish are full and will not eat anymore. Then offer them live mosquito larvae. They WILL eat the mosquito larvae, even while excess frozen blood worm lies on the tank floor, but not vice versa.
I suspect fish favor live Black worm vs live White worm. If you have both then try both and let us know.