Player's choice. Both can be done nicely so it's your playing style that will be the main factor. Most players might lean towards the STR route as it will increase your damage per hit, and more hitpoints (esp with Body Development).
Furry is a great skill. When maxed out it offers an insane increase in DPS for a few seconds, which is great for bosses and tough enemies. It pairs well with Duel, too. It's a great skill for light and 1-Hand warriors. In contrast, it can hinder 2-H warriors. As you note, AGL 6 is a pretty big investment. The extra defense is nice for two-hand warriors, the extra trap dodge is nice, too. But you won't get any increase in your attack from AGL using 2-H weapons.
But again, that 75% increase in DPS is brutal.
In contrast, increasing your STR from 4 to 6 will give you +5 damage per hit (2-H Expert), will give you 36 hitpoints (Body Development, and more as you gain levels).
You increase your current skills or consider adding a new skill. Maxing out Heavyhand will give you the best over all damage output as it will add damage to every attack.
Passive Skills that might go well with your current build: Bloodlust, Massive Critical, Precision Strikes.
Active Skills: Charge.
Charge is useful as it gives you another high-damage skill while your Whirlwind Cool down is active. Charge can also take you safely across traps, or through an enemy to escape, re-position, or to attack the healer in the back.
Whirlwind and Charge don't play the nicest with Duel (can break the duel count). Charge and Whirlwind can break your duel count and force you to start over.
With 2-H characters I tend to avoid Duel and use Charge and Whirlwind. It can be tricky to get the aiming down, but Charge + Whirlwind is a high damage combo. Max Charge does +160% damage, Max Whirlwind does +120%. Land both hits and it's a total of 480% damage (including the 100% of the base attack).
I leave Duel for my 1-H characters who are going to be in longer fights.
