knock-knock: Either caliber choice offers plenty of options. But its always best to try before you buy. Likely the best way to decide which works best for your needs, is to handle and especially shoot(if possible), samples of either caliber/grip type. Likewise, different barrel lengths. Do any of your local gun clubs host cowboy action? If so, this is a good place to get practical hands-on experience. Somewhere among the mix, you should find the one that you prefer.
Also: What drives your choice? Will the revolver be used for casual range plinking? Or see competitive use? In which case, as old303 suggests, reloading costs also have to be factored in.
.45 Colt and its offspring: .45 Schofield and Cowboy .45 Special are going to be more expensive to reload than say, .357 mag....and especially .38 Special. So... if your budget is limited...or, you intend to compete...those component costs can really add up. Not so much for casual trips to the range. Though still more.
As far as authenticity goes, or just plain fun factor...there's nothing that beats the big Colt cartridge in that department. But, the most practical choice overall....at least in my opinion...is .357 magnum.
Al
ps: From firsthand experience: The short-stroked Smoke Wagon is a good choice, if serious competition is your goal. Otherwise, the Patron will serve you well. Though it can certainly hold its own against all comers. Also, be sure to check out the various Pietta models. Equally as good as Uberti(in my view).