You can't beat the Vaquero for cowboy action shooting. For pure speed, though, the best choice is the .38/.357 New Vaquero with the 4-5/8" barrel. It is lighter and quicker handling than the original Vaquero, and the slightly shorter barrel will clear leather quicker.
In practice, there is almost no difference between the 4-5/8" and the 5-1/2" barrels. Muzzle velocity is so close as to be indistinguishable, and the sight radius difference is also negligible (and irrelevant for CAS, which is all about speed not accuracy). Still, the 4-5/8" may save a few hundredths of a second and make it slightly less likely that you'll hit the holster lip with the end of the barrel while reholstering, which can cost you anything from a 1/2 second to a match disqualification (for dropping a gun).
The .45 Colts are neat, but not very practical for cowboy shooting. To be competitive in cowboy shooting, you need to shoot very low recoil loads. That's a lot easier with a .38 than a .45, just because of the weight of the bullets. In addition, the .45 is already a huge capacity case, and getting small powder charges to burn consistently is a problem. Lots of people start out with .45s in cowboy shooting, but they almost all end up with .38's if they get serious about competing.
The New Vaquero won't take the +P .45 Colt loads that the traditional Vaquero will, but only you can decide if that really matters. My way of thinking is that if you want to hunt with your handgun, get a hunting handgun like a Blackhawk with decent sights. If you want to cowboy shoot and plink, get a New Vaquero and stick to reasonable loads. Also, your hand size and personal preference matters. Some guys like the thicker, heavier traditional Vaquero. I don't. To me, they handle like bricks. I've always preferred the feel of a Colt, and the New Vaquero handles just like a Colt.
Disclosure: I've been cowboy shooting for several years, and do pretty well at it. Despite what I said above, I compete with 7-1/2" barrel 44-40 Remington M1875 replica revolvers. Why? Because I like them. Also, I compete in Frontier Cartridge (black powder only), and pure speed isn't as much of an issue as it is in the bigger categories like traditional and modern.
Still, if I was trying to win, I'd be shooting 4-5/8" New Vaqueros in .38.
