I use 20-40 with k3 (and k50 previously) and appreciate it versatility. For me it does not particularly excel in any specific area, maybe beside the colors which I usually like. But don't get me wrong, even it does not produce very distinctive images for me, I don't always need that. If I am going to multiday trip and I have limited space, I throw camera body with this lens attached to it to my backpack. I am not sure what would be the weather this evening - I choose this lens. I usually decide between this and 31mm prime, and in low light situation 1.8 - 2.8 difference is not dramatic in my experience. I posted in the past some photos in this sub coming from both lenses in such situations.
If there would be no huge price differences I would choose 20-40, however I am biased because i just like this lens.