Ok, I own several Rugers in 45 Colt,, as well as a FA in 454, & an El Dorado in 45 Colt. I also own other .45 Colts.
To get to the meat of things.
DougGuy has put forth some excellent info.
Ruger first put out the Old Model .45 in 1971. It was a Blackhawk. (Sorry Drm50,, it wasn't in the 1960's.) The OM's stopped in 1973,, and the New Models were introduced. It wasn't until 1993,, that the Vaquero's came along,, and they were FIRST out using the same frame size as the Blackhawk. The New Vaquero came along in 2005,,, and it's frame size was scaled down, (as noted by DougGuy.) It along with the later Flattop 45's,, use the "mid sized" frame,, and can handle hotter loads than the older Colts & other guns made many years ago. (Again, good reference DougGuy.)
There have been several variations in barrel length & grip frames to go along with the .45 Colt made by Ruger. You have several options,, including the ability to swap grip frames to your liking. All of Ruger SA handguns use the exact same screw hole pattern. Some of the NM's no longer have the cut out in the mainframe that mates to the g/f,, so there may be a need to modify the grips, etc,, but it's easy.
The point to all this is the fact that there are variations within the Ruger line that can fill most desires. Many of the Colt Clones out there,, often fail to be as versatile.
And as noted,, a Vaquero has fixed sights,, and can sometimes need tweaking to get a load to shoot to POA. I once swapped into a nice Original Vaquero that the owner knew it shot low & left. (He failed to tell me, until I confronted him later.) But,, I was able to tweak it by opening the rear sight channel a little, and regulating the ammo.
And if you wish to know the difference in frame size,, the Original Vaquero's (on the Blackhawk frame,) will have a 2-digit prefix in the serial number. The New Vaquero's will have a 3-digit prefix.
I have shot many of the Colt clones & many felt & shot good. But the long term wear & tear, as well as re-sale is not as good as a Ruger.