Considering the gap between the Rokinon 8mm (I have that lens and love it) and the 18-55 WR, I vote for the 15mm. I hope you'll bring the kit lens, being able to go out in bad weather and shoot is such a great advantage for Pentaxians.
Suggestion: You could rent both lenses and see which one you prefer. I have been renting lenses frequently in the past year, it's been very helpful in determining my next purchase. I use
borrowlenses.com and I have been very happy with their selection of lenses for Pentax, as well as the price and service.
BTW, both places are gorgeous, you'll have no end of subjects. I look forward to seeing your shots.
Also, you might not have enough room (or desire) for a tripod, but consider taking a beanbag. That little bit of stability coupled with the cameras anti-shake, will let you get some great long exposures.