I am in a similar situation to you.
Im thinking about an s2 but worried, like you are that its not much of an upgrade from the mixtrack. However I think because of its good build quality and reliability and extra features then the mixtrack on the s2, albeit not that many, I could get a good few years use before I feel I need to upgrade. Also with the offer on at the moment, its hard to turn down.
Im not sure I want 4 decks to DJ with which would make the s4 not really worth the money for me.
Like previously suggested I would think about getting the Kontrol F1 or something similar to increase your options.