Ilford states ilfosol 3 is for their lower ISO films at 1:9 and 1:14 for others and economy. Is one better than the other for Delta 400? My subject was low key edge-lit images that are mostly black.
Dilution is pretty much a what-ever-works-for-you thing.
More dilute means it takes longer to develop, some people like that, some don't.
I will say that, unless you are doing testing as rigorous as Koday or Ilford might, you would probably be hard pressed to tell which is which between two negatives as long as the negatives were developed to the same contrast.