I am just building my mournfangs right now, going to play some games tomorrow with them. As much as I really want Great Weapons on them, going from a 2+/6++ to a 3+ is a huge difference. If you took 9 s3 wounds on those saves, you're taking 3 wounds with the 3+, but only 1.25 with the Iron Fists, meaning there is a good chance that Mournfang will live to strike. An ogre can't attack much when he's dead.
If you're going to use the Mournfangs to hunt monsters though, then I definitely say go with the Great Weapons and Banner of Eternal Flame. Many monsters are striking at S6, so reducing your 2+ to a 5+ or your 3+ to a 6+, not a huge difference, but wounding on 3's with S6 (vs T5) or wounding on 5's with clubs could be huge.
For attacking basic infantry, the Ironfist is king, but if you're fighting against high S/T units a lot, such as Chaos Warriors with Halberds, monsters, etc., then the Great Weapon could have an advantage. Personally, I'm going with the Ironfists because I have Ironblasters to solve the other problems.