With the introduction of the Magical Archive, we are now able to see real values of the spells, runes and other things in the game, so answering the question, here is the comparison between Thunderstorm and Avalanche:
Today, both Avalanches and Great Fireball Runes have 50 Base Power, 5 more base power than Thunderstorm and Stone Shower, which have 45.
However, with the upcoming Vocation Balance, all 4 runes will have their base power 50, which should always have been.
Now it will be very simple:
Will you hunt a Fire sensitive creature? Use GFB.
Will you hunt a Ice sensitive creature? Use Avalanche.
Will you hunt a Energy sensitive creature? Use Thunderstorm.
Will you hunt a Earth sensitive creature? Use Stone Shower.
Before, it was a bit complicated because you needed to do some maths to know if using a weaker rune with 5 less base power would justify going for creature's higher sensitive element. In some cases keep using GFBs and Avalanches were better than using other two runes, but now after the balance you won't need to do that again.
