Pentax HD Pentax DA 15mm f/4 ED AL Limited Lens

Recently I have decided to buy a wide angle lens in order to improve my landscape pictures, some architecture and street pictures. Forasmuch I own Pentax cameras with K bayonets, first, I was trying to make decisions between zoom lens Sigma 6-18 and Pentax12-24. In the end I have decided to buy a fixed focus lens (anyway I do own only one prime lens by now) for my APS-C camera as some kind of compromise because I know that next year full frame Pentax camera should be introduced.