I was also considering between P20 Pro second hand or brand new Mate 20 which is quite similar in price, even used vs second hand.
End up going for P20 Pro as I think the camera "should" be better on P20 Pro, and the easy to use form factor.
But Mate 20 advantage is the Kirin 980, but for me it is only going to be my work and camera phone so I thought P20 Pro will do, and I prefer the design of the P20 series triple cam over the Mate 20 triple cam set up.
But I think both are equally good, as Mate 20 non pro gives you the wide angle capability.