Whether Ice Floes is better than Shimmer is a bit of a moot point I think.
This late in the expansion, Blizzard isn’t going to make any real changes before the pre-patch.
And once the pre-patch hits, one of the changes we know about is Blizzard is reducing Shimmer from 2 charges to 1.
Since Shimmer will be pretty much useless with the upcoming nerf, Frost Mages will all probably end up with Ice Floes by default anyways.