In my opinion it doesn’t matter that much between the buffs and the Delaware. If you skin instead of pluck the Australorps can work fine too. When you pluck you leave pin feathers behind. With dark feathers those are easy to see but you get a prettier carcass with light colored birds.
How important is size to you? To some people it is extremely important but half the chicks I hatch are female so half the chickens I eat are female. There are only two of us so size is not an important criteria, although I select my roosters partly for size because bigger is nice when you butcher.
At what age do you butcher? How fast do those strains of buffs, Delaware, or Australorps pack on meat? Non-hatchery stock doesn’t tell me anything. Different breeders have different goals and different knowledge on how to breed to reach those goals. If you butcher fairly early how fast they put on meat instead of first growing a skeleton could make a difference. I suggest you talk to those non-hatchery breeders and see what they can tell you about those birds, how fast they put on meat and whether or not they go broody much. Some Buffs, Delaware, and Australorps go broody a lot, some not so much. It’s something a breeder can control by what criteria they select for their breeding stock.