Including replicas I have several 4 3/4", one 5 1/2", several 7 1/2" and one 3 1/2" (Thunderer). I certainly love the looks and balance of the 4 3/4". I guess the question has to be asked as to whether you are asking for CAS use or not. As Deacon mentioned, the shorter barrels are more convenient. But when I'm not shooting conversions, the SAA's I shoot most are 7 1/2" .45's. I have a method of drawing and reholstering with authentic leather which is easy but not too fast with butt forward. But I also have a set of strong side holsters that are angled inward somewhat. They look a little gamy which I'm not crazy about but let me draw them pretty easily. Of course buscadero rigs are good for that (if you can make yourself wear one, haha).
So, if you only have a 4 3/4" I'd say you NEED a 7 1/2 as well, and then you can decide if you NEED a 5 1/2 also. It's really all about personal choice, very subjective.