Howdy
The Single Action Army was made in three standard barrel lengths, 7 1/2", 5 1/2", and 4 3/4". There were other lengths, but those were the standards. Historically speaking, the original barrel length in 1873 was 7 1/2". After 20 years or so many or the original revolvers were sent back to Colt or government armories for repair and refurbishing. Most of those guns had their barrels cut back to 5 1/2" at that time. Many of these guns were reissued to artillery units when the Spanish American War broke out in 1898. For that reason, the 5 1/2" barrel length is unofficially called the Artillery Model by collectors. The 5 1/2" barrel was the most popular barrel length with the SAA, selling more than any other length.
I shoot a pair of 2nd Gen Colts in Cowboy Action Shooting. One has a 7 1/2" barrel, the other is 4 3/4". I think the 5 1/2" barrels look boring, but that is just me.
I can tell you from experience that it is difficult to draw a SAA with a 7 1/2" barrel from a high riding strong side holster without getting your elbow tangled in your armpit. So I wear my 7 1/2" Colt cross draw on my weak side.
As far as increased velocity from the longer barrel is concerned, yeah there will be some, but you will not notice it. Yes, the sight radius is longer, and that should help a bit with accuracy, but to tell the truth I do better with my 4 3/4" Colt than I do with the 7 1/2". If I cared about speed, which I don't, I could tell you that it takes longer to pull the long barrel from leather than it does the short barrel. But I don't care about speed.
The real reason to shoot a 7 1/2" SAA (or clone) is because they look so cool.