Ruger SAs, and ...the rest
I have Ruger Blackhawks in .45Colt/.45ACP, and in .357 Mag. Also a Super Blackhawk in .44 Mag. Built like tanks, cpable of taking any max load (and for the Rugers the max is often higher than other guns), AND they have adjustable sights. This means after I decide on which load I want to shoot, I can adjust the gun to hit so that point of aim and point of impact are the same. This is important to me.
I had a Ruger Vaquero (.44 mag) for some time, and wound up trading it for the Super Blackhawk, because I wanted a gun with adjustable sights. I don't play the cowboy game currently, and when I do get around to it, I will get another Vaquero (or maybe even a Colt, or a clone) and tailor the ammo for the gun, but until then, I want my guns to hit where I aim.
Many years ago a friend had a .357 SA gun (I can no longer remember the exact brand, Cattleman, I think) and with its fixed sights it shot perfect point of aim at 20 yards with 158gr .38Spl ammo. Every .357 round tried in it shot high and left (about 4 inches at 20yds). If you decide to go with a fixed sight gun, remember that it will only be on target for one load, at one range. For eveything else, you will have to learn the hold off, in order to hit. Cowboy games are played at short range, so this might not be as big a problem in the game as it is when shooting for real, but it is something you will have to live with.
I have my own issues with the rules for cowboy shooting, like not allowing certain guns in certain classes, even though they are chambered for 1873 era cartridges, and also allowing ultra light loadings (I think they should require the full power factory loads of the era, in smokeless equivalents), and a coupld of other things, but what the heck, it is only a game, and if I want to play their game, I will abide by their rules. Gives me an excuse to get some more guns, anyway.
If you want the best shooting (most practically useable) for the money, get a Ruger Blackhawk. If you want something to play games with (with less real world untility) get a Vaquero, or something else that looks and feels like a Colt SAA. If you get one cheap enough, you can spend the money you save on your costume! Or, maybe, ammo! If you spend it on ammo, you just might be able to learn where your fixed sight gun actually shoots, which may or may not be where the sights look. Either way, good luck, and have fun. After safety, that's what it is all about.
Oh, yeah, the other plus of the Ruger(s) is that they can safely be carried with six rounds loaded, unlike the original Colts and some of the clones.