I have a couple of answers:
1. It's pretty much "flip a coin" time. The gun and shooter matter far more than the cartridge, in this case.
2. There are minor differences in the grip frame and the cylinder, but you won't really notice them in terms of performance. The Super Blackhawk Hunter has integral scope bases, if you're into scoping single action revolvers.
3. Bring a .453" bullet with you when you check out the .45 Colt. If the chamber mouths are too tight to let that bullet be pushed through with little or no pressure from your fingers, that gun may give you problems with lead bullets. That's because the barrel's grooves are almost certainly .452".
Same with a .44, if you can't slip a .430" diameter slug through the mouths, it will need to be opened up. I had a .357 mag Blackhawk with chamber mouths that were as small as .355", with the largest just barely mic-ing at .357". The barrel was cut for .358" slugs. It didn't like cast bullets, but it shot jacketed okay. Bring a feeler gauge and check the cylinder gap while you're giving that gun the once over, you might as well check everything at once.
4. 5.5" barrel. Revolvers are supposed to be handy and light.
Either caliber will serve you very well, if the gun you pick is well made.
