They've both had their strengths and weaknesses in each raid, it's pretty boss by boss so it's hard to say it's just a spec thing.
If you're not a mythic raider, your choice of Frost or Unholy is really down to whatever is more fun to play. At the end of the day, you can take a small amount of comfort knowing that no matter how bad you think Frost is, it will still outperform a Ret Paladin.
They both have 4 buttons and prioritize RP dumping during burst; difficulty wise- they're pretty equal. 7.0 Unholy is far, far more forgiving than BoS 6.2 Unholy was.