By no means am I an expert on this as I'm fairly new to the darkroom myself. Knowing that, I would think that the 2.8 would help you to make focusing easier. Being a faster lens it will allow more light when set at 2.8. After you finish focusing, close down the lens a couple of stops. At that point I don't think one will differ from the other because 5.6/8 will be the same on either lens.