Even as a warrior-no especially for warriors I find the Atronach stone much better. I already hit the armor cap long ago, so resisting 50% of spells over 25% of all spells seems like a much better bet.
A Breton mage might prefer the Lord's stone. Armor for the unarmored, and resisting 50% of all spells instead of the racial base of 25%.
If you like to fight magic, then go for the Atronach. If you like staying alive in general, go for the Lord.
What race are you? The Altmer has the power to regenerate magicka plus bonus base magicka.