Taking the opportunity of being reminded about my own thread 1 year ago (thanks @UncleVanya), I would like to report that I actually pulled the trigger not long after opening this thread (LBA LBA

), and sold the 12-24 shortly after. I apologise for not being able to provide comparisons photos, because first I'm very uncomfortable doing that work, and secondly to have infinity landscape with enough details to do comparison I have to drive out of the city. So here is my subjective "judgement":
11-18 +:
- It is sharper and crispier. It was down to my copies of course, but for sharpness I think no technical comparison is needed, if you see your photos on a large screen you will realise the 11-18 is sharper, especially at wide end
- Much better build quality. Not a limited type but the 11-18 is very well built, feel great in my hands, and also it's operation (focusing, zooming, etc). This feeling when using a lens does not contribute directly to image quality, but it has quite an impact on my motivation to take photos, personally
- Adding to good handling is silent focusing
- Internal zoom. It's not technically internal zoom, but it looks like an internal zoom lens (the front elements movement is very shoot, and only inside the outer of the lens
- WR. I think for a landscape lens WR is an important feature.
- Wider. 11mm is wider than 12mm
- Very good flare-resistance for such a zoom
- Could be use with FF from like 16-18mm. Even at 15mm with significant vignetting I still prefer this lens to the Irix 15mm which I had at the time. I eventually sold both the Irix and my K1-ii so this no longer applies.
11-18 -:
- An used 11-18 is still at least 2 times more expensive than a 12-24
- It is heavy, really heavy, and
- It is big big, and its hood is ridiculously large. It takes a lot of spaces in my bag, and in fact for a trip it eats up the space of at least 2 other random lenses than I could usually bring
- Limited zoom range: it's not possible to leave this lens as a default lens on my camera, due to 18mm being too wide for the better part of my shoots. 24mm could do otherwise
- My cameras are the KP and KS-1, with both of them the lens makes the combination very front-heavy, quite clunky to carry
12-24? Basically the reversal of above points:
- It is lighter, smaller, and cheaper. I do miss its size
- In exchange, build quality is nothing to rave about
- 12-24 = starting to enter normal range territory, so it can be used as a default lens
- Sharpness is a bit inferior to the 11-18, but I feel the color is as good
- I don't remember it's flare resistance ability
Conclusion is already known: ideally you should have both, with the 11-18 for more serious shooting, long trips, rain, etc, while the 12-24 for more casual ones. If wide-angle photography is just a complementary part of your habit, you can live perfectly fine with the 12-24, however if you take a lot of wide-angle photos, eventually you would upgrade to the 11-18, that's inevitable.