Depends on your goals with the bike to some extent. In general, the SS tail with stock seat will be a little more comfortable because of the thicker padding, but the Superbike Tail will "hypothetically" help you go faster.
Most people feel the superbike seat gives you better feel on what the bike is doing, and better feedback. Most SBK tails are actually thinner and narrower around the seat area as well, and allow you to move from side to side just a little bit easier (IMO). Personally, I greatly prefer the SBK tail, even when going slow
Most fast racers and professionals prefer the SBK tail when it is available. Look at the AMA SBK grid, or the front of a WERA expert race. That being said though, there is always an exception to any rule though, and I've seen really fast guys prefer the SS tail.
Most Coaches and CR's I do body kits for prefer the SS tail, because of the sheer amount they are ridding, and aren't stressing it if slows them down .25 seconds to be more comfortable.