It's too early to gauge spec/class strength fully as people are still getting ToS gear, working out best talents, etc. As it stands all classes are pretty close in terms of dps. Mythic logs aren't very reliable, and if you look at heroic there is less than 10% difference in frost and unholy, which is probably larger due to the "best" players switching to unholy.
Unless you are competing at a high level just play what you enjoy.