I like the big bores and will always choose one over the .357. The .45Colt can cover the .357's spectrum and much more.
While it is common perception that the Colt SAA is a weak design that must be held to SAAMI-spec pressures, this is not really the case. SAAMI pressure standards are held in deference to the very old guns chambering the cartridge. Post-war Colt's, replicas, S&W's and the mid-frame New Vaquero can all be loaded to 20-21,000psi. Dave Scovill designed the RCBS 270SAA bullet specifically for the SAA and he likes to drive them to 1100fps with 2400. Yes, in Colt's.
And while some really punishing 357's are out there (some of the 180gr stuff is awe inspiring) again I wouldn't shoot it in an SAA or clone.
This is no issue whatsoever. Except for the very early Colt SAA's, which were not properly heat treated, the SAA is one of the strongest .357's available while still being of appropriate size for the cartridge. Its cylinder is larger in diameter than any mid-frame double action like the venerable 686 or GP-100. Nope, there is no reason to worry about hurting a .357 SAA or replica.
A better solution than either cartridge is the .44Spl. In mid-frame guns like the Colt SAA, replicas or mid-frame Blackhawks (factory and custom), it can be loaded to higher pressures than the old .45 due to its thicker cylinder walls. The 1200fps Keith load is no problem but we tend to prefer the 950fps Skeeter load for everyday use. It is a much more efficient cartridge than the .45Colt at these power levels with a greater margin of safety. However, the .44Spl must be handloaded to see it's true potential.
In large frame Rugers everything changes and the .45 can be loaded equal to the .44Mag, up to 32,000CUP. The Winchester and Marlin 94's can be loaded a little hotter than this level.
In custom five shot Rugers, it can be loaded to 50-55,000psi which renders the .454 unnecessary. The late model Winchester 1892's and replicas are close to this strength range (up to 45,000psi). They are the strongest of the pistol cartridge leverguns.