7 1/2" was the original barrel length. 4 3/4" was considered the "civilian" model, due to its popularity among regular citizens. 5 1/2" was made popular when refurbished SAA's were cut down from the 7 1/2" length and issued to Artillery troops during the Spanish-American war.
The reason for the long barrel was accuracy and history. (Long barrels were the norm for military handguns). In my opinion, the advantage of the long barrel is all in the shooting, and none in the carrying. The 7 1/2" hangs out there, and that extra muzzle weight can be a real help in stilling the front sight for accurate shots. If you've ever packed a full sized gun, well, they always seem to let you know they are there.
Conversely, the civilian version is all about fast action and ease of packing. For all practical purposes, the 5 1/5" is a slightly elongated civilian model.
In standard, non-magnum chamberings, the short barrel adds little in muzzle blast, and handles exceptionally well. You do loose some velocity, and sight radius, but the gun itself is better suited to being a companion.
I think the best place to start is by honestly evaluating your use for the gun. If its going to the range and back in a bag, then the full size will be fine. If you intend to actively carry it, then the shorter lengths are best. If you ever intend to have it save your bacon-which it can and will, then the 4 3/4" is in my opinion the superior choice.
