Freeze is superior to Blizzard II in every way. Blizzard II is a point blank AoE (centered on you) and you are probably not standing in the middle of a pack of mobs that will also be using their own AoEs. Freeze instantly puts you at max stacks and does more damage than Blizzard II. You can completely ignore Blizzard II at any level that Freeze is available.
Your MP refresh rate is based on how many stacks of Umbral Ice you have (see UI/AF breakdown several posts previous). You only get stacks if your spell hits something, and a spell that generates a single stack will just set you to 0 stacks if you have any stacks of the opposite element, so switch elements with a Tier III spell (Fire III / Blizzard III / Freeze) before using a lower spell.