The reason that TG will be slightly (a little more than it was in T11) ahead of SMF in T12 in equal gear for several small tradeoffs and two more noticeable advantages:
1. Higher 4-piece frequency
2. Increased Crit% + Deep Wounds Damage Range
It is still viable to use SMF for the first part of the tier until the necessary gear is acquired (either at least 1 Zoid's 2H Sword or 4 piece) to make TG outperform consistently. Note that it will be much easier to acquire and upgrade the 1H weapons
since they are BoE, and since the 2H weapons will also be desired by Ret paladins and DK's increasing competition for good 2H's.
With full BiS gear
TG will be 7-9% higher DPS than SMF due to extra sockets and superior weapon choice. Getting a combination of 2 of either Zoid's or Sulfuras's quickly will be difficult unless your loot system favors you highly over other players.