according to the patch notes the proc rate on unholy is unchanged. so, based on some dudes study off wowhead:
Unholy is not a PPM enchant, it uses a chance on hit % formula that scales with weapon speed.
this enchant won't proc as often as fiery does on 1h weapons, so for rogues duel fiery is probably going to stay at the top, because a small increase in dmg per proc and the curse effect probably isnt worth it procing much less. although i could see this replacing fiery on 2h, because of the way it procs it will be closer to fiery's proc rate on slower weapons .
in spite of that, the curse effect decreases white damage given by 15, thats like half in a lot of cases (1h melee swings, ranged attacks) so in arenas or on defense this could also be useful to decrease the damage being given, in addition to the burst.
edit: lol i thought i clicked the 3.3 rogue thread but this is fine too.