FrogReaver
The most respectful and polite poster ever
I agree with level 1-3 if we are just counting single target damage. Cleave is likely much better at this stage for damage, though it can be a bit DM dependent on how they position their enemies. You also lose out on useful Masteries like topple that don't up your damage, but instead increase your Battlefield control and possibly aid your allies damage.For any barbarian from 1 to 4. But always for Beserker.
Level 1 to 4
Greatsword is 2d6 + rage
Nick is 1d6 + 1d6 + l 2 rage
=+ rage damage (+ 2d6 for Beserker)
Level 4 with feats like GWM, PAM, Sentinel that all can give extra attacks, I think it's much harder to say this. So much will be assumption based that i don't think the question can be definitively answered.
For levels 5-8 that puts the Nick Barbarian down .5 damage (not to mention feat options acquired at level 4).Level 5+
Greatsword is 4d6+2 rage
Nick is 3d6+ 3 rage
= so your down 1d6 weapon damage, but up a rage damage
That's a 1 time per turn ability, that already has nearly a 100% hit rate with 2 reckless attacks. The third attack does add a bit of accuracy here +2.2% assuming 60% base chance to hit. That's not enough to make up for the 0.5 gap.(+2d6 for Beserker).
That said, for all intents and purposes they are dead even from 5+ without feats. It's just feats likely favor the non-dual wield barbarian.