In which case the normal force will be greater if we consider two identical blocks, one resting on a flat surface, and the other resting on an incline?

For the flat surface, we know that N = W. For the incline, due to the angle of the incline, N < W. In fact, we can see that N = W cos(). Since Cos() can only be between 0 and 1 we know N must be less than W.