Myself, I always get the Lord Stone (+50AC and 25% magic resist). I intend to grab the crown and get the Atronach stone for magicka absorbtion. Not being a magic user I don't care about the drawback (reduced magicka regen) but will be nearly impervious to magic-based attacks. (also am a breton and wearing two items of 15+% magic resist already).
I read on the uesp wiki that the spell absorbtion percent is the CHANCE that the spell will be absorbed, not that 50% of the spell will be absorbed. So 50/50.
Did you get the Solitude Shield version of resist magic?
I have also tended to favor Breton for the resist magic ability. I am playing as a Wood Elf right now to tone down just how OP I can get. With the Breton I go with the first two Resist Magic perks from the Alteration tree, for the Wood Elf I went with all three, so one less perk to be spent elsewhere. Also, with the Breton, all I needed was one slot for magic resist enchantments, for the Wood Elf I needed two. For both builds I am sticking with the Guardian Stones for the faster/easier leveling. The Lord Stone does have some nice perks, though. I used it earlier, but by late game you don't really need it.