play the spec you have the most fun with, trust me. if there is you as frost and a UH guy in your raid, you both will be neck and neck but beat each other on some fights by a bit. Do not fret.
these stupid sims are calculated by a machine. perfect button presses. 0 latencey, not moving or getting any debuffs, etc. Use the trinket section to see what you should gun for first. the rest will sort itself out in 4 weeks when raids open.
I put up plenty of infor on my time as frost on beta doing mythic 5+'s. I was fine. everything was fine.