Canon 1200D has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Nikon D3200 has a 24.0MP APS-C (23.2 x 15.4 mm ) sized CMOS sensor and features Expeed 3 processor. Nikon D3200's sensor provides 6MP more than Canon 1200D's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 1200D and D3200 sensor size comparison.
Canon 1200D and Nikon D3200 have almost the same sensor size so none of them has any significant advantage over other in this area.