I have the Sigma 18-50 f/2.8 (non-macro) lens but not the 18-55 WR. I do have the DA-L 18-55 kit lens though. Anyway...you said that your reason for possibly switching is:
Originally posted by crazy_oam because I want smaller and lighter equipment to carry out there
If these are the only two criteria for you, then you'll certainly acheive both with the 18-55 WR. If you also want/need a WR lens to go along with your K-5 WR camera, then this also makes sense and a definite plus in favor of the 18-55 WR lens.
But if you need the faster, constant f/2.8 aperture across the zoom range, then obviously the Sigma 18-50 is better in that regard. And although I've not done detailed tests between my Sigma 18-50 and DA 18-55 kit lens, I think my Sigma is sharper optically, although the kit lens can hold its own when stopped down to f/8 and used outdoors. But if you're shooting indoors in poor lighting, the Sigma will be a better option.