Along with Magefist, Frostburn is a pair of gloves directed towards spellcaster use. While Magefist gives a large boost to mana regeneration, Frostburn gives a very large boost to mana itself (which stacks with other items that give a percentage-based increase of mana). While Frostburn gives no boost to skills and Magefist gives a bonus to Fire Skills, choosing between the two is a matter of taste.