Both Nikon D3200 and Nikon D3400 have 24.0 MP resolution sensors but Nikon D3200's sensor is APS-C (23.2 x 15.4 mm ) and Nikon D3400's sensor is APS-C (23.5 x 15.6 mm ).
Since Nikon D3400's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon D3200.
Another difference between these two cameras is that Nikon D3400's sensor lacks anti-alias (Low-Pass) filter. 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 D3200 and D3400 sensor size comparison.
Nikon D3200 and Nikon D3400 have almost the same sensor size so none of them has any significant advantage over other in this area.