Barrel length is a matter of preference and practicality. If you want to be a quick draw kind of guy, you'll probably want a shorter barrel. For long range shooting and or hunting, a longer barrel is better.
The Vaquero has channel sights, and most Blackhawks have adjustable sights.
Calibers are a question of personal preference as well as economics and availability. .357 is a great one because you can shoot cheaper .38's as well. There are some convertible models such as .45/.45acp. .357/9mm.
I've owned two early .44 OM Vaquero's, a .45 convertible Blackhawk, a .357 Blackhawk, a Single Six which is .22LR/.22Mag, and a Blackhawk in .30 Carbine.
If you want a revolver that is just plain fun, get yourself a .30 Carbine Blackhawk. They only come with 7.5" barrels and if you ever shoot one, you'll know why. It is a real hoot with lots of noise and flame with that one.
It makes a pretty good light game gun too.
BTW, convertible models just come with two different and interchangeable cylinders, easy stuff.
Hope this helps.