I've been a cowboy action shooter for many years. I still shoot two or three matches each month. I've played with, shot and loved single action revolvers for sixty years. And I have been inside of about everything on the market, past or present. I won't call myself an expert on single actions, but I am no novice either.
First regarding caliber, if money for ammo is not a consideration, then .45 Colt is a wonderful caliber. But even if you reload, it will cost about twice as much to shoot .45 as it will cost to shoot .38 special. How much you shoot or plan to shoot also comes into play. I know a lot of guys who shoot around 1000 rnds per month. For them the savings are very significant. While it is very satisfying to shoot normal strength .45 loads, you should know that you will be shooting against others with smaller calibers and lighter loads. Because of recoil you will never shoot as rapidly as these fellows. So if winning is important to you, buy .38/357 revolvers.
Regarding revolver choices, here is a brief overview.
The STI is a wonderful revolver. If you can afford a pair of them (you'll need two) then by all means, get them.
Colts are not as pricy as the STI. I have several sets of them. The current production Colts are of excellent quality, but they will still need a light action job. The big attraction to me is that they are Colts, and that matters to me. However a good clone can be made to shoot just as well and the STI a bit better perhaps.
The USFA's are excellent clones. Fundamentally they are just as good as Colts, but they aren't Colts. However, if I wanted a pair of excellent cowboy guns at an affordable price, I would go to Longhunter Supply and order a set of USFA Rodeo revolvers complete with his action jobs. For the money they are an outstanding value and they will be nigh perfect when you receive them. The only thing I don't like about them is the brushed finish, blue and nickel.
For a little less money, Longhunter will sell you a pair of tuned Ruger New model Vacqueros. They will serve you admirably. I don't shoot them because they are modern guns in my view. But they are good guns and if you aren't into the history aspect, you will be very happy with them.
I am very partial to the Great Western II revolvers sold by EMF. Their parts are generally interchangable with Colt parts, and they are well made guns. They should be quite adequate right out of the box.
There isn't a thing wrong with the Uberti guns from various distributors. All but their "special" models such as the "Evil Roy's and Smokewagons, will require an action job for optimal performance, but they will be slick and dead dependable once they are tuned. My only complaint is that the case hardened finish should have a little more color.
You will note that I have mentioned getting your revolvers tuned several times. This serves two functions. First, because the revolver will feel so slick and will be so much easier to manipulate, you will shoot it better. Second, you will put less wear on your guns. Friction will be minimized. I shoot a pair of revolvers that are over 100 years old. They are as good as the day they left the Colt factory, maybe better!
There is no truth to the myth that Colt and clone revolvers are subject to breakage. Of course, if you don't know how to handle them or if you abuse them, they will be less dependable. But they rarely break. I will grant that I have to replace a hand spring or a trigger bolt spring in a heavily used gun every few years. That costs me less than $5.00 and about ten minutes of my time. And any fool can make these replacements in 15 minutes. Most serious competitors simply replace these springs annually so the problem never comes up, and these guys shoot tens of thousands of rounds each year.
Rugers use all coil springs. They seem to never fail. But even the Ruger benefits greatly from tuning.
Of all the guns mentioned, STI is the only one that isn't going to need a tune up because of the absolute precision of their machining. The Uberti Smokewagon and the Evil Roy models as well as the Great Western II revolvers are tuned by the distributors prior to shipping.
Regarding barrel length, it really doesn't matter. I routinely shoot them all. However, and this is just a personal thing, I find that uniformly across all barrel lengths, the .45, 44-40, .44 spec. and .38-40 have slightly better balance that the .38/.357. I think the shorter 4.75" barrel offers the best balance in .38 caliber. In the larger calibers, the SA revolvers balance well with all three standard lengths.
In summary, even if you buy the cheapest set of SA revolvers that you can find, they are going to perform quite well with a good action job. I am partial to Colts, but you don't have to buy Colts to get a fine revolver.
I can't comment on the Beretta and Taurus revolvers as I have no long term experience with them.