Both have good sides to offer. You need to consider:
Highlander C2 and C3 offer VERY good skills for both PvE and PvP. Cross Cut, Skyliner, and Moulinet from C2 are indispensable for dealing damage. Cross Cut applies a bleed debuff, (this isn’t a weak bleed either - It does 1/3 the damage of Cross Cut every second for 10 seconds), that makes Skyliner do double damage… If you go C3, Skull Swing drops enemy defense to zero for 10 seconds…and Vertical Slash does additional damage based on enemy size + the amount of defense reduced from Skull Swing + the amount of debuffs currently on the target. They all hit very damn hard too. Wagon Wheel is very useful in PvP as it knocks up enemies…particularly ones that aren’t using Pain Barrier. Allows you to setup a combo with Cross Cut to Moulinet.
Barbarian gains multiple damage buffs and has some of the hardest hitting attacks in the game right now that are AoE. Helm Chopper is a very nice attack…I’ve been hit by it in a duel before and it does very consistent damage, has a long hitbox, and is an on-demand stun. Cleave nets a short huge boost to crit rate while dealing heavy damage.
If you’re going for a beast 2-hander, I recommend going both. Highlander C3 then to Barb C2 in the end…you’ll do an insane amount of damage mixing the 2 together.