The FA 43 was the first prime that I bought for my K-1. I've never used it on any of my APS-C cameras. I bought it just to give me something extra in terms of color, microcontrast, and rendering that you'll never get out of the DFA 28-105, with it's more generic imaging. I use the 43 mostly for landscape and rarely for it's fast aperture. The FA 43 might be a little sharper than the DFA 28-105, but the differences are not all that significant. The reason to get the FA 43 is for the unique look it provides---the ambiance, the more life-like, or at least piquant and interesting, rendering of detail, or for fast aperture use---not for any extra sharpness, which is perilously close to imperceptible in any case. And of course it can be useful for fast aperture use. With a lens that expensive, one needs to be comfortable with that specific focal length. I find the "true normal" focal length of the 43 very comfortable to shoot at. I could photograph at it for long stretches of time, which minimizes lens changes. But for those who don't find the focal length comfortable (either to wide or not wide enough), there are better options.
A rare (for me anyways) shot with the FA 43 wide open: