Howdy , Shuttleman, and welcome!
Two guns vs. one -
If you are going to shoot SASS (the most popular and widespread organization), you will need two revolvers, as most stages require 10 rounds to be shot, and you are only allowed to load five in a revolver (even if you've got a transfer bar!). It's a safety issue.
If you shoot NCOWS, they have a "Working Cowboy" class which only uses one handgun.
Caliber choice -
There are many reasons for choosing one caliber over another. If you shoot the SASS category "Classic Cowboy", your pistol and rifle caliber must be .40 or over; that lets out .38/.357 for that category.
If you are recoil sensitive, .32s and .38s recoil much less than .45s. In addition, many of the top shooters use .38s because the lower recoil lets them get back on target faster.
If you shoot one of the black powder categories, you may find that one of the bottleneck cartridges (38-40 or 44-40) seals the chamber much better and gives you less blowby than a straight-walled cartridge (.38 or .45) with the light cowboy loads we shoot. There is a "cool factor" involved in the bottleneck cartridges, too...
Some folks (including me) like the bigger boom and recoil of the larger calibers.
I shoot .38s because of economic reasons. My wife Lorelei shoots SASS too, and we go through 500-1000 rounds a month. I don't reload, and .38s are a heck of a lot cheaper than .45s.
I will give you the same recommendation that I give all new shooters: Go to a local match before you buy anything! Most folks will be falling all over themselves to let you try their guns. There are many manufacturers out there besides Ruger, other pistols besides the Colt Model "P" (Remington, Colt open tops, etc.), and other calibers besides the .38 and .45. Get other folks' opinions, and "try before you buy."
And welcome to the most fun you can have that isn't immoral, illegal, or fattening!!!