I would personally go a different ulti but for your question go with ice comet for a few reason. Ice comet has snare potential which means more time in the aoe dot left behind and does more upfront damage that combines with the warden passive to boost even more. If you want to pump out more though, go with shooting star. The ulti from hit combined with shimmering's major heroism is a great combo. I've tried both and liked ice comet out of the 2 though.

Permafrost with a good shalk rotation wrecks face when stacked ontop of corrupting pollen