+4 faster casting is definately preferred over using protection, as you can cast about every spell without too much interuption. it is difficult to obtain without building a suit specifically for it.
protection will help if you find yourself getting interupted a lot trying to cure poison or heal with close wounds, if you do not have and fast casting properties. you will be casting even slower with protection active, and it also means you will be frozen for some spells even longer. and the decrease in resist means even more poisoning, greater stat decrease, longer paralyzation and curses.
i try to avoid using protection, as it's a hinderance in developing more effective stategies against powerful foes (and pvp), but i'll use it in pvm when fighting a large groups that can interupt frequently, even with faster casting.
