Truthfully, Sadris, I'm not entirely certain of the exact mechanics behind the chance for such spells as Shackle Undead and Mind Control to end before the full duration has been reached; as a player myself, however, I'm willing to offer an educated conjecture. Although this is not an "official" answer by any means, I would be inclined to believe that +hit would merely affect the initial cast of the spell, and not the periodic checks to see if it will break early. Items which reduce one's opponents' spell resistances, however, should increase the chance for the effect to last longer. Given that the initial cast is important as well, however, I don't see any harm in focusing on both statistics. Good luck to you! :)
Ce sont ces fenêtres qui m'appellent...