Derek, I have made 15 windsor chairs using two different reamers. I started out with an 11 degree reamer made by a guy named Fred Emhoff IIRC. He was a machinist in New England and made spoon bits also. It was okay and cut nicely. Then I read about the a narrower angled reamer, as I recall, championed by Curtis Buchanan. He was teaching chair making down in South America and showed them how to make the reamer. So I made a six degree reamer out of an old saw blade and turned the body on a lathe and put a T handle in it. The blade had teeth on one side and was ground and honed, ala scraper style on the opposite side. When Elia's six degree reamer became available, I bought it and have used it since. It is super!

To answer your question, I think the six degree taper is easier to seat and offers a much stronger joint, however it will split the mortise open easier also, due to the mechanical advantage of the narrower angle. But by the same token, it is this advantage that adds to strength when driven home. I doubt, however, there is that much difference as an end result due to engineering of the chair.

I guess what I'm trying to say is, the six degree M/T joint just feels better. Bottom line, either will do. As C. S. has famously said; "There are six ways to do most things. t Two of them are good. Two of them are bad. Two of them will get you by.