Is there a difference in stability between a tail downforce (cg ahead of ac) and a tail upforce (cg behind)?
No, cg ahead of neutral point is stable, cg behind it is unstable.

Neutral point is the centre of pressure when both main wing and horizontal stabilizer fly at the same $C_L$. If the cg is just a bit ahead of neutral point, the tail still flies with positive lift. If it is ahead of the centre of pressure of the main wing, tail flies with negative lift.

The further forward the centre of gravity is, the bigger the difference between the coefficients of lift and therefore the stronger restoring force. But note that since it is second order negative feedback, the restoring force creates phugoid oscillation and if that is too fast, it will be difficult for the pilot to stop. So the stability has to be just right.

This is still true in canard configuration, but there the forward surface that flies at higher $C_L$ in stable configuration is the canard.