I have no experience with Buffs but there was a wide variation in my WH's. I think it is possible you could end up with some nice looking, correct to standard ducks in either breed. Even the premium hatcheries/breeders have ducklings of different qualities. The only way to know would be to keep them till they were all feathered out, I'd think. My WH's had a lot of color changes as they grew older . . . I think though that you want to sell the ducklings, so you wouldn't want to keep the extras that long.
Buff ducks probably don't have the gold and silver phases that the WH's have, so they might be a better choice just because of that. I'd keep what you like the best!
I have my WH"s for eggs, so I don't care much how correct to standard they are, as they are doing a great job laying.