Frostburn provides a small benefit of about 14% more damage (60% cold vs 40% cold) and the chance to freeze which can be detrimental to grouping enemies. I would recommend the Mantle of Channeling over Frostburn since it provides a superior offensive and defensive capability. Alternatively, you can consider something else like the Ess of Johan.
You can find more ideas in my other thread: List of things to put in the 4th cube slot!