[Brightcolours]

flash output depends only on aperture and subject distance, nothing else, so with a fast aperture you need less flash power thus less recycling time for flashes, also with less intense flash output and with the help of the diffused evening lights you have much less shadows, which is more pleasant in portraits, and here I am relating my own experience, maybe there's another explanation, but that's what I saw  

I still don't get it.

If you put the 5D at ISO 400 and the 30D at ISO 100, and you shoot with f2.8 on the 30D and f4 on the 5D, what happens?