Well, the Porpoise goes 7 knots faster than the the S-Class on the surface and when you recharge your batteries you will still go on two propellers. Two extra torpedo tubes and four extra torpedoes are a nice addition as well. And last but not least it can go deeper.
But, well, sometimes statistics aren't just enough to convince someone to take a new boat. I'm sticking with my Porpoise now even though I've been given opportunity to upgrade to another model twice. I just like the boat.
I liked the S-Class a lot too, but it was simply too slow on the surface.
