Erco's

Looking to get some Erco's.
I have found someone in the States that has some in stock and is willing to ship them to me (UK) pretty much immediately.
The only thing is...... he only has 5 5/8" Rears in stock (getting 3.5" zero o/s for front). He says he can get 6" Rears but they could be some time before they're in stock.

What i hoped somone could tell me was, what's the major difference?? (apart from the 3/8" in size!!!)

If it's just the 3/8" difference in width, then surely you wouldn't even notice when they're on the car, so why do they go to the trouble of manufacturing a 5 5/8" width and a 6"?

Something in my head questions that there is more to this, but unfortunately, my head doesn't hold the answers!!