I had the same question, although slightly different (should I have gotten SS before PiT?). It makes sense to me that a sequel to the game would have the same gameplay concept, etc, and I'm more of a totally-new-idea guy, although I've been told to disregard the sequel factor and get both of them, since they were so good.
Anyway, it's up to you. I haven't played SS personally, so I can't vouch for it, but I liked PiT and if you want more of the same (?), go for it.