The K-S1 is brand new in the market (therefore also the high prize) without much dedicated reviews. Always good to wait for reviews. For brand new equipment a premium prize is paid, you could check the K-3 prize evolution for reference.
A lot of people here in that forum seem to drive their DSLR to the shoot out. That is not common practise for everybody. :-).
If size is what matters, it appears a better choice compared to the K-50. How small it is you could check here (comparison to the Canon EOS 100D and to the Olympus OM-D E-M10):
Compare camera dimensions side by side Compare camera dimensions side by side
The main competitor to the K-S1 seem to be the Canon EOS 100D (or EOS Rebel SL1 how it is called in the US) and the Nikon D3300. And in that respect, the Specs of the K-S1 are better in a lot of areas.
The canon has received a lot of very positive attention with a Gold Award on depreview. It may be worthwhile to read that review to get a feeling which way Pentax probably would like to go with the K-S1:
Canon EOS 100D (EOS Rebel SL1): Digital Photography Review
"With everything a family photographer is likely to want and little left over to intimidate, the Canon EOS Rebel SL1 may be Canon's best-targeted digital Rebel to date. As the top end of the Rebel line - currently the T5i - added more and more enthusiast features, the camera got bigger and more complex, and the SL1/100D stands as a suitable alternative without much compromise where it matters. Indeed, it currently stands as the better alternative for those who want to shoot in live view mode, and for anyone who thinks they'll want to shoot a movie now and then, thanks to Hybrid AF II. Image quality is also uncompromised, as we've come to expect from the Rebel line.
Its smaller size and lighter weight make it easier to pack and carry, meaning the Canon SL1 is more likely to be used, and its fuller feature set helps it stand out for those dissatisfied with their smartphone shots. The good news is the SL1 is a real pleasure to shoot, with ergonomics good enough for small to medium-size hands, and snappy performance, both when shooting and reviewing photos. The touchscreen makes menus and playback work like a cell phone, making it familiar to more users than would normally be the case. The only element missing to help it compete in the smartphone-dominated market is Wi-Fi, which can be added via an Eye-Fi card, which is supported in the SL1's menu.
The SL1 isn't necessarily the perfect answer to the mirrorless camera, as smaller-sensor offerings abound in cameras from Nikon, Pentax, Panasonic, and Olympus. Smaller sensors have their disadvantages, but one of their advantages is they can deliver equivalent focal lengths with smaller lenses. Though the SL1 is itself smaller, its 18-55mm kit lens is noticeably larger than kit lenses on cameras like the Olympus E-PM2 or Nikon V2. Still, for a great many looking for SLR quality in a smaller package, the Canon Rebel SL1 delivers both with confidence and grace, and offers the advantage of a larger APS-C sensor."