I've not used either but I have a K30 which is basically a K50. It's a great camera, for sure, but I've read some glowing reviews of the K-S2 which has an extra 4MP and no AA filter (for more detail). Therefore I would think seriously about the K-S2.
As for the lens, well the K-S2 kit 18-55 is not great, so I'd be looking for a body-only. Then maybe look at a Sigma 17-70, a used one if funds are low. I know that doesn't answer your question really, but it might be worth considering options other than the two you mentioned. I'd always go for used lenses on fleabay, and experiment with some old manual ones too. I've not used the Tamron but for the price you could buy the older Pentax 18-55 WR and another lens. All depends on what you want to shoot.
Good luck and enjoy your photography