The sl is essentially the predecessor to the dreamline series. The 2 main differences between the current models and the streamline is the new smooth twist x barrel and the interchangeability of parts that allows you to change the dreamline platform to other versions. In regards to the barrel system I would say that the accuracy gained is minimal, as the streamline and other past fx models with the original smooth twist barrels were still really accurate guns capable of sub half moa groups at 50 yards. The new dreamlines allow you to buy kits (eventually as FX takes forever to release products they promised) to change from bullpup, rifle, compact rifle, etc. If you are sure you want to keep the classic rifle look then I don't think it would make much of a difference. They have a few other minor changes such as a newer reg and bigger magazine, but those don't make much of a difference. The sl and dl were not created with the intention of shooting slugs, they lack the power to push them fast enough to take advantage of the accuracy. However, fx recently released a power plenum for the dl that would up the power, making slugs a viable option for the dl. However I am not sure if you can fit it onto the streamline.