FrogReaver
The most respectful and polite poster ever
This is wrong.
Greatsword + Hunter's Mark, 3 rounds, STR 18, Extra Attack:
3*(6d6+8) = 87
1d8 main weapon + 1d6 off weapon, 3 rounds, Hunter's Mark tag round 1, dual-wielding 2 and 3, STR 18, Extra Attack:
(2d8+2d6+8) + 2*(2d8+4d6+8) = 86
Even after three rounds, the greatsword is a point ahead of the dual-wielder with a d8 main. The dual-wielder pulls ahead in Round 4, which is the last round of many combats, likely mopping up.
Additionally he's assuming that you never killed that enemy and needed to move the hex/hunter's mark to the next enemy. If that happens then you never catch up.