I have both (bought the Bisely Vaquero for particular black powder matches that specified fixed sights).
If you are only going to buy one and want to shoot it, by all means go with the BlackHawk. The adjustable sights are a godsend when you try to zero the gun for a particular bullet or load.
Typically, fixed sighted guns can be "regulated" for only one specific bullet and load. That can be a real pain. Further, even then, it isn't guaranteed.
My Bisley Vaquero was a lucky buy for me. Right out of the box, it hit to point of aim at 25 yards with my chosen black powder charge and the 250 gr Lyman 452664 Cowboy Bullet. However, if I wanted to shoot lighter bullets it would need work with no guarantees.
My SS Bisley Black Hawk .45 Colt/.45 ACP Convertible can easily be adjusted for either 200gr, 250 gr, or 285 gr bullets with a simple twist of a screwdriver. I can shoot from mild to wild and hit where I need to hit with a quick and easy adjustment of the rear sight.
Dale53