These lines should perform similarly, considering they both have the same name. The thing is, they are very different. I was surprised by the results. As I said, I usually fish the Rio version. I find it versatile because it is accurate at all distances, picks up off the water for a recast or second shot well, and can turn big bugs over at all distances.
My initial reaction to the SA line was that it seemed too heavy and slowed the rod down. But how could this be? The head weight is substantially lighter. After fishing it for a few hours, I picked up more line than average and set the fly back down accurately at longer distances. I also was shocked by how well this line made quick short shots.
This line has a much shorter head, so it is lighter. However, it has most of the weight up front. While the Rio line has a delicate front taper, with most of the weight being towards the back of the head. This means you must have more line out to load the rod so things go slower.
By the end of the three weeks, I preferred the Rio line but was very happy fishing the SA. Some people like longer, more delicate lines, and others like short and quick. The SA Tarpon line makes a great all-around tarpon line, especially if you can't cast as far or are planning to fish in the mangroves.
What does this all mean?
When buying a line for a specific application, do a little research or call one of the shop's guys, and they can help you make a quick and accurate decision.