For me it would not be a descision between 35 mm and 43 mm. I would like to have a 35 mm on APS-C as well as a short tele, preferable a 50 mm but the 43 mm would be excellent option too.
I bought the 35/1.4 Sigma last november as an APS-C equivalent of an 50/2.0 for my K-70. It is an very special lens. Although very sharp in the middle, often lacking contrast and to be stoped down to 4.0 to get good sharpness across the frame. It is an excellent lens, yes. It is very fast, yes. But you have to know its flaws and strengthes to get best results. In practice I can fully support the test opticallimits did with this lens on the Nikon APS-C. Link below. Astonishingly the 35/1.4 seems to perform much better on fullframe, see second link.
If one does not need the shallow DOF (which was my main target) I would fully hearted recommend the Pentax 35mm/2.0 HD over the Sigma 35/1.4. It has very good performance over the full frame from wide open, i.e. on 2.0 to 2.8 it performs better then the Sigma, it is much lighter and compared to its performance dirt cheap. In favour for the Sigma are DOF (just one stop!), built quality and silent AF. AF speed on the Sigma is in my opinon not much faster then on my shaft driven lenses.
Sigma AF 35mm f/1.4 DG HSM | A ("Art") (Nikon DX) - Review / Test Report Sigma 85mm f/1.4 HSM DG ART ( Nikon FX ) - Review / Test Pentax SMC-FA 35mm f/2 AL - Review / Test Report - Analysis
The Sigma dock did not influence my decision. If the Sigma would not have had focus on the spot I would have returned it. For reliable focus using the center spot only is mandatory on the K-70.