All the armored horses have essentially the same speed / maneuverability ratio. See on this
chart how the lower difficulty horses have varied speed and maneuverability, but from the destrier on up it is not varied at all. Speed is always a point or two less than turning ability and that's it. A very boring progression.
I'd rather see armored versions of the courser and arabian. Like so:
Charger
hit points: 120
body armor: 45
difficulty: 5
speed: 44
maneuver: 37
charge: 38
Cataphract Horse
hit points: 120
body armor: 40
difficulty: 5
speed: 37
maneuver: 46
charge: 34
Also, there is no reason to have 4 horses at difficulty 3. I suggest taking the Steppe Horse (which no one uses anyway) down to difficulty 2, and the Sumpter to difficulty 1, each with appropriate nerfed stats. Then having 1 point in riding will not be entirely pointless.