all i know is that its better to take the ice imbue instead of frost nova.
the frost nova is 1 second in which you give the opponent frostbite and with 50% probability, you give him freeze (if he isn't immune).
With the imbue on the other hand, you can hit him 2-3 times in 1 second, give him freeze and frostbite (lower probability) but give him damage as well
so i'd take the first 2 frost novas to lvl 1 at most and take ice imbue instead to fight any monsters stronger than champion, and any unimmune opponent in pvp.
Freezing opponents is really an advantage if you can debuff them aswell