Hey, I thought I would revive this thread as I just bought some swivel knifes and probably bought too much stuff because I couldn't find this particular info on the web. I'm fairly new to leather and thought I would try some different knives. I bought some Barry King's because they were available here in Canada (Longview Leather in Alberta).
First I bought a 3/8 barrel and a 3/8" angle blade. As for the barrell, I think the advantage of a thin barrell is that you can turn it a little easier so it is really nice for tight carving, like details of 1/2" lettering or flowers and leaves in small areas. However, i find that because I have to squeeze my hand together more, I fatigue quicker than I do with a larger barrell. I think the ball bearing swivel also helps as it just feels better as you turn it.
As for the blades, I did not like the angle blade at all. I found that because I was holding the knife straight up and down all the time, it put my hand in an awkward position. I don't know how to describe it other than to say that it made it feel like the knife was too long for my hand. Also I found it was hard to get nice decorative cuts because only the very tip of the blade was in the leather. Because the cut was so thin (on 2-3oz), it was difficult to bevel it after too.
Later, I bought a 1/2" barrel and a 5/8" barrel Barry King at Longview.....I know it sounds crazy but at $40 plus blade, I don't really think they are that expensive......anyway, as expected the bigger barrels feel very nice and comfortable and they turn very nicely in your hand, but you just can't cut as tight of a curve as you can with the 3/8". That being said, the difference is not huge.
I also bought a few different blades....all of them straight....I bought the 3/8" Thin, a 1/4" Thick, and a 1/4" Thin.....I wouldn't get the thick and thin in the 1/4", because for me, the differnence is marginal on 2 to 3 oz leather, but maybe it will make a bigger difference on 8 to 9oz. I actually wanted the thick one in a 3/8, but Longview didn't have them in stock, I think the 3/8" might be noticeably thicker and may give nice thick decorative "feather cuts". I really like the straight blades better because you can vary the angle to do different things with it. Like if you need a nice straight line or long sweeping curve, just tilt it upright and get more of the blade in the leather. If you need a tight turn, tilt it away so that it rotates better. BTW, I think I could have done just fine without the 1/4" blades at all, the 3/8" looks a lot bigger but functions almost the same.
Anyway, that's my 2 cents. I had trouble finding this info before I bought the Barry King's and probably over bought because of it. My recomendation if you can only have one.......a standard 1/2" Barrel with a 3/8" straight blade......maybe a thick blade depending on what you are doing. I guess it says something that this is the format for all the basic Swivel Knifes at Tandy.
Cheers!