Nikon D3400 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Expeed 4 processor. On the other hand, Pentax K-S2 has a 20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME MII processor.
Another similarity between these two cameras is that both Pentax K-S2 and Nikon D3400 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the D3400 and K-S2 sensor size comparison.
Nikon D3400 and Pentax K-S2 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Pentax K-S2 has 20% larger pixel area (18.37µm2 vs 15.28µm2) compared to Nikon D3400, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.