Thanks you very much guys!
So I decided to post u finally my decision, to let you know and to thank you all for your opinion and shares. Indeed, the 35 f/2.8 macro from pentax is really one of the best I saw for very close ups at that focal length. I just love all the examples I saw here in this forum and elsewhere. I was at the beginning afraid it could be very slow not in macro mode and you told me it's instead doable with the quick shift setting (only for DA lenses). Its weight and its size would fit every bag and every photographer; moreover, since it's macro, it's very sharp edge to edge. And can fit the everyday use, shifting quick from macro to a wideish angle mode.
Despite all, I decided for the sigma 30mm 1.4.
I know: it's not pentax (and I chose the k-5 for the lenses it could have); it could have focusing problems (as notices from some users here in this forum which can be almost totally recovered by the fine focus tuning), and could be not as sharp as the 35 macro, especially at the border. Moreover, if focuses at most at 40cm far and it's quite heavy to carry.
Although, I decided for it since everytime I was having a look at its pics, I was amazed by the object separation, in how it shines and, well, I've to admit that, about the f/1.4 sharpness (I already have a 1.4 aperture in the 50mm, but better having two than none, right?). Somehow, it's not that far from the FA31mm. In addition, it has a quite fast AF.
I'm not saying that I won't regret the macro possibility offered by the 35 limited; though, maybe in the far future, a 100mm pentax or 90mm tamron will let me suffer less...
Thank you all for the support! This forum is really a must before or while doing anything with a pentax!