If you're looking to get the sharpest photo possile, the K-S1 wins hands down. It lacks an anti-aliasing filter. Get a 50mm prime & you're gold.
The K-50 can also put out some amazing photos, but if you're looking for absolute sharpness, the anti-aliasing filter is not going to help out.
Here lies the problem. Sometimes you get "moire" in repeating patterns like fabric, bricks, stucco, etcetera when no aa filter is present. The K-S1 has a selectable aa simulator that can help out. You just need to figure out when you'll need to use it, but that reduces sharpness a bit.
---------- Post added 08-12-15 at 07:26 AM ----------
By the way, unless you really need a camera now, I suggest you wait until "Black Friday" this year. My guess is that there will be some insane deals on whatever Pentax K-S1, K-50, & maybe even K-3 cameras are left. You can probably get a Pentax K-3 flagship that will take care of sharpness with a selectable aa filter, weather sealing, & has a ton of features, but then that might be overwhelming for a beginner.
Pentax Price Watch - PentaxForums.com
^ Make sure to look at the "Pentax Price Watch" section of this forum. Sometimes you can find some incredible deals in there. There was a recent deal for a blue or olive Pentax K-50 with a flash, 4GB eyefi SD card, 18-55 lens, & 50-200 lens on eBay for only $399. They basically gave them away at that price. Other deals pop in once in a while, but you basically have to act fast.