Jack,
Personally, I would go for the S4.
The main reason for this being that its an “expandable” hardware piece.
What I mean by this, is that its built in sound card will allow you to expand your setup.
E.G. Connecting turntables with timecoded vinyl or CDJ’s with timecoded CD’s. (tonecode in Serato terms.)
There are ways to Map out the S2 and S4 to work with VDJ.
In my opinion, if you’re going to be using your hard earned money to acquire a controller, you might as well spend the few extra bucks to get a controller you could use in the long run.
As far as a learning curve, it all depends on you.
Work hard at your passion, and you’ll be able to accomplish unlimited things!
Keep on spinnin’ !