Stairs - material being used

We are having a 2 story house built. On the majority of our stairs, it is not possible to use real wood due to the size of the stairs being over 12 inches wide. The builders have used different materials on stairs (with no apparent reason – I asked).

On some of the stairs it is treated plywood. On others it is particle board that is also used for the floors upstairs.

Which is better? Does it matter that they are inconsistent?