I like the look of the 5 1/2" but I went with the 4 5/8" because all the stuff I read suggested to me that it would be slightly faster. I only have one so I'm not equipped to compete - really I bought it because I'd just finished reading Kuhnhausen's shop manual on the Colt SAA. I really like how simple the mechanics of the gun are.
The original was 7 1/2" and I wasn't going to go that long so it was a toss up for a bit which of the shorter lengths I'd go with. In reality I think I'd be just as happy with the 5 1/2" as I am with the 4 5/8".
If I get a second I may go longer just to see if I can tell the difference (which, of course, means I'll end up with a third at some point
).