The solution!
Base: +8 armor, +1 Dex, -5 armor check, 35% arcane spell failure
Alloy: +6 armor, +3 Dex, -4 armor check, 25% arcane spell failure
Mithral: +4 armor, +5 Dex, -3 armor check, 20% arcane spell failure
The armor bonus, combined with the maximum Dexterity bonus, adds up to +9 for each armor type. And mithral no longer increases your base land speed, so it isn't quite as obvious a choice. The existence of both feats is intended to allow you to choose medium or light armor, just as a normal character would. And, hopefully, this leaves them more or less equal.