The Outbound lines are integrated lines - integrating what is essentially a shooting head with a shooting line. These lines are intended for distance casting with a quick delivery - just pick up and shoot. The Outbound lines have longer heads of around 37 feet I think. You need to cast with the entire head out to get the best out of these lines and some anglers have difficulty with that. The Outbound Short lines have 30 foot heads which many find easier to use. The Outbound Short lines excel at making long casts with big flies. Either model would be great if you have a need for a lot of long distance casting. With the thin running line it is not a great roll casting line for distances beyond the length of the head and it is more difficult to pick up for a standard overhead cast if you have more then a few feet beyond the head outside the rod tip.
I really like the Rio Mainstream lines for the price and have been using a Mainstream Saltwater line for freshwater bass angling as well as salty stuff for 5 years.