Blizzard is powerful, but it takes an extra turn to cast, and can potentially be countered. At the point where you get -1 casting time you can cast two Fireballs for one Blizzard, and Fireballs will be cast instantly (no chance to be countered; kill enemy units one turn faster, i.e. before they can attack). I do generally prefer Water magic to Fire magic, but to some extent this is a matter of taste. The -40% mana cost for tactical spell (Mantle of Oceans?) from Water is awesome, but Fire mages can do damage. Focus is a nice cheap spell for offensive mages. Burning blade is not that exciting but is still quite a handy buff for any combat heroes you have, especially if you have a lot of fire shards. Flame Dart and Burning Hands are ok, especially at the start of the game. Heart of Fire is a very important spell for making your Trained units effective later in the game when armour values get silly.