I'd go with something like this:
Light Armor: +4 AC, Max Dex +4, 30' speed, Skill Check Penalty -2, Arcane Failure 10%
Medium Armor: +6 AC, Max Dex +2, 25' speed, Skill Check Penalty -4, Arcane Failure 20%
Heavy Armor: +8 AC, Max Dex +0, 20' speed, Skill Check Penalty -6, Arcane Failure 30%...