I just can't decide which one to use with my robe Mage. The 25% magic resistance and 50 armour sounds good but I'm not sure I need over 300 armour rating. Then the Atronach with 50% spell absorbsion and +50 Magicka, with the 50% slower Regen. I am leaning towards the Atronach because my robes will be enchanted with Regen.
So what do you think is better, I just can't decide.

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.

If you're wearing robes, then Atronach is your best choice.

Beware though. If you take the Atronach Stone power with the Atronach perk, your 80% magic absorbtion affects your ability to consecutively summon creatures every time.

Apprentice.

Atronach has 2 problems.

1. It is a gamble... 50% chance is good, but it can fail when you really need it.
2. The absorb your own summons bug.

Therefore i would go with the lord.

Lord stone all the way. My mage is unarmored/unarmed and between the Lord stone, the Atronach perk and magic resistance perks in alteration, and Ward absorb in restoration I never run out of magicka. I'm level 49, and play on adept, and honestly the 80 ar I have from the Lord stone and my shield in enough that I rarely ever use flesh spells, or dragonhide unless I'm up against a few falmer warmongers or deathlords. Just run in and shield bash with grand healing refilling my stamina every couple bashes.