We're talking about someone who doesn't know either. Right now the overwhelming majority of players are Frost. It doesn't matter whether any one person is specifically better at Frost or Unholy, for a new player picking up one of the specs from scratch the one that is "better" at the moment is Frost, all tools and all data seem to point to it.
Going from
http://raidbots.com/dpsbot/ the results are showing Frost as being #6, and Unholy as being #19.
Going from simulationcraft
http://www.simulationcraft.org/510/Raid_T14H.html the results show 1h Frost being 500 DPS above 2h Frost, and 2h Frost being about 6k dps above Unholy.