I have both regular RCBS and the cowboy dies (yes they have a carbide ring), I like the cowboy dies because the crimp die seems to be made for cast bullets more than the older RCBS dies I have, in which the crimp die seems to work better for jacketed bullets. The cowboy dies I have seems to be more of taper crimp, and doesn't roll the mouth of the case as much. I like the cowboy dies better only because of this. I also agree they did not have to do the heat treating thing, and if you want to get rid of the brass lock rings, you can replace them. Cowbody dies also come with two different size expanders (one in 454 as you asked about), in case you need to use slightly larger diameter bullets. I think the bullet seater is a slightly different shape as well, for cast lead bullets. I bought the cowboy set, but should have checked into just buying a decent crimp die, maybe just a bullet seater....but if you don't have any dies in 45 colt and are going to use lots of cast bullets I guess you might as well get the cowboy dies. Note...I have not used the 454 exapander as I buy most of my bullets in 452. But it could come in handy I suppose if you needed to use 454 bullets in one of your guns.
Hope this helps some.