Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Have a hard decision.

Epic Weapon Specialization
Does it give +6 damage or only +2 due prerequisite is Weapon Specialization and Greater Weapon Specialization (that I’m going to get)

nortlander hewing should then be better to choose (+6 damage)? Even if I get 4 attack instead of 6.

Epic weapon specialization gives only +2 MORE! So that brings you to +6 with your chosen weapon. (Specifics: The character gains +6 to all damage dealt using the selected weapon. This feat can be taken multiple times, but applies to a different weapon each time. This bonus replaces that of Weapon Specialization and Greater Weapon Specialization.) As for Northlander hewing, honestly, I'm not sure, look at this this way. If you're a barbarian with lots of str (or any other high str class, but barbarians are beefy muscle boys) with lets say Manticore greatsword (1d6 bonus piercing dmg) and also whirlwind frenzy, its not bad, its actually not bad, you're not dependant on crits like you would be with a falchion or a scimitar, and as far as I know NH does not, oh well, look at this (Updates from 1 June 2019: Maximize damage effect and AB penalty removed.) Wack. I also realized I've not been helpful so far, sorry Bhor.

NH I'd say wins anytime.

Between essentially between giving you your str mod to damage in addition to your normal str bonus (allowing you to punch people for 20 + 1d3 dmg!), it also gives you some neat random procs depending on damage type.

High STR with great weapon wants IPA over NH (especially with Frenzied Zerker 5)...sure you can use both, but then you get both feats negative too.
High STR with Sword and Board wants NH, the STR bonus is not changed based on one hand or two hands on the weapon.

Low STR build doesn't really want either, but would likely go PA/IPA over NH since it is a flat bonus (i.e. Halfling Barbarian with 13 STR, max DEX finesse if that is still a thing due to NWN2 bug)

As to NH over Epic Weapon Spec, 2x6 = max of 12 damage, but depending on your AB you are likely going to miss the last attack, possibly the last 2. If you assume 5 hits with EWS that is 10 damage, if you are getting 6 damage per hit with NH and still likely to land all 4 then the 24 damage is a clear winner. Just remember NH has downside, which EWS does not.