For the OP's benefit, let me just explain in simple terms what we are debating about autofocus (AF). @arv has pointed out two improvements in the autofocus performance of the K-S2 over the K-50:
1. the screwdrive works faster. The point of this is that on most lenses for Pentax cameras the focus mechanism is driven by a screw powered by a tiny electric motor in the camera. (The exceptions are lenses - generally more recent models - labelled as DC, SDM or HSM, which have a motor in the lens which drives the AF.) So a more powerful or responsive motor in the camera makes a screwdriven lens focus faster. The K-3 is said to be a step up from either the K-50 or K-S2 in this regard. Where is matters most is in heavier screw-driven lenses (e.g. my Sigma 400mm f5.6) and lenses with a long focus throw (such as macro lenses).
2. better AF senstivity. The K-50 autofocus is rated to work at -1EV whereas the K-S2 and K-3 work down to -3EV. That means one and three stops (respectively) below exposure value (EV). Don't worry about the technicalities - the point is that the K-S2 or K-3 will focus more reliably in low light - even with the AF assist light that each has. How much this matters will depend a lot on how often you are likey to shoot in semi-darkness without a flash (or, as arv says, using a ND filter for a long exposure). And how elastic your budget is.
For a full comparison of specs between the K-50, K-S2 and K-3, see this table:
Pentax Cameras | Pentax K-50 vs. Pentax K-S2 vs. Pentax K-3 - Pentax DSLR Comparison - PentaxForums.com
One important factor is the feel of the camera. Try to handle each one before you buy. The K-S2 has a smaller grip than the K-50 which may or may not suit you. The K-3 is the papa bear of the family, with extra bulk and heft. Personally I find the K-30 (basically the same camera as the K-50) fits my hand perfectly, but everyone is different.
Don't sweat the choice too much. They are all fine cameras. You can't really go wrong.