I have both the Ricoh GR and DA 15. They are both good for hiking. If bringing a single camera+lens I prefer the GR because 18mm is a more generally useful focal length than 15mm(*). When 15mm works, though, it works very well.
The DA 15 offers better color and flare resistance than the GR. That's not to say the GR is bad, it's just that the DA 15 is even better.
The GR offers better sharpness. GR corners are very sharp even at f2.8. The DA 15 corners are weaker, but acceptable especially considering that most landscapes are stopped down a bit.
(*) 15mm is significantly wider than 18mm. It's only 3mm but you can be surprised by how much that matters at the wide end. Note that I avoided converting to the oft-cited and oft-confused "35mm equivalence" because your GR and K-3 are both APS-C sensors.
DA 15 Example (mounted on K-5)
Ricoh GR Example