I'd take the Colt just because I think it will hold up better to sustained firing of .357 mags. I'm also a sucker for the Colt finish. That said, the Trooper III was not a popular gun because it was not as accurate as other magnums.
With a 6-inch barrel, the S&W will give you better accuracy (probably) and it will last a good long while if you shoot a lot of .38s and heavier bullet .357s.