I disagree with double ice block being greater utility on at least heroic KJ vs a fire mage with the execution belt. The following just relates to the heroic encounter but:
Strictly because of the meteor in phase 3.
A fire mage is excellent for kiting the meteor.
First, they can maintain 100% dps because KJ is usually sub 30% and scorch and all other spells needed for optimal dps can be cast on the move.
Second, casting scorch vastly increases our run speed, and allows us to move much faster than frost can, while maintaining 100% dps uptime.
Third, there are already plenty of soaks for the big armageddon, the tanks can take it during intermission, and you only really need 1 or 2 max after that. That extra ice block is not gonna make or break it for most raids.
At the very least, they are equal. Frost does not have more utility for that specific fight imho, and I've kited that meteor every time I've ever downed the thing, and played both.
Also frost doesn't have second life, and blink healing. When I play that boss as frost, and I have to kit the orb, my dps dies. I also have much less passive healing, and no second chance if I die to a stupid mechanic, so my opinion is fire is much better on that fight as far as utility for the raid, and ease of use for the player.
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Let me say something about this thread. Last tier, everyone here would have you believe fire would suck for this tier(ToS), and it ended up(imho) the best spec in mythic. My reasoning per boss: when frost won, it barely won, when fire won, it could win big.
Now, the predictions are even closer. I would say fire may be the go to spec, but we shall see.
https://www.warcraftlogs.com/statistics/13