A low-wing plane is often less stable than a high-wing plane, but other differences come into play; such as, how much dihedral it has and what the airfoil is like.

In the case of the Tiger, the airfoil is thick and it has a good amount of dihedral. It is my favorite choice for training students.

As for the H-9 PTS Mustang, it has clear plastic boots that fit over the froward edge of the wing which increase the wing area and provide more lift. It is a good trainer as long as you are on a buddy box with an instructor - which you should do regardless of the plane you are flying.