If it was only between those 3, for me it would be a no-brainer: Heighten. It does lots more, for lots more spells, in lots more situations.
My feat list would be pretty similar to yours, except with Heighten in "must-have" list, and Shield Mastery in the "maybe" list. I generally have enough protection when I'm shield-turtling without Shield Mastery, and most of the time, I'm not carrying a shield at all, because there are so many useful caster-oriented weapons to be carrying instead.