Cowboy shooters tend to go for the 4 5/8, as it's the fastest in and out of leather. .357s aren't at all unpleasant, though you might lose a smidgen of velocity.
The 5.5 doesn't make a noticeable difference in speed from leather, and some people like it for balance or a touch more sight radius. It's a game of tiny increments over the 4 5/8, in both regards.
The 7.5 is a different animal. It's considerably slower into and out of leather. It's bulkier and more annoying on the hip. The extra sight radius really does make a difference in my group sizes, shooting supported. Unsupported, wobbles seem to affect all lengths equally. It gets a few more fps out of any given load, and also slows the recoil cycle significantly.
FWIW, my .357s wear 4 5/8 barrels, and are good general purpose trail guns, in addition to their primary mission of winning cowboy matches. My .44s wear 7.5 barrels, however, and are better as dedicated hunting revolvers than for just knocking around the woods.
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