The Duel vs. Heavyhand debate hinges on the six hit rule, so to speak.
With Heavyhand III, a player gets +5 for each attack. That means in six hits, the player gets an extra 30 damage.
With Duel III, a player gets +2 per consecutive hit, up to +10. (0,2,4,6,8,10,10,10, etc). That means after six hits, a player gets an extra 30 damage.
Thus, for enemies that are killed in less than six hits, Heavyhand provides more extra damage. For enemies killed in more than six hits, Duel provides more extra damage. The Duel count also resets if you break combat (cast a spell, use an item, step too far away, etc). So a lot of times, Heavyhand's consistent damage bonus will outperform Duel. The caveat being that in tough boss fights, Duel's extra damage will really shine.
I once kept combat logs for a few different dungeon runs to go back and see which skill actually contributed the most. Overall, Heavyhand was head and shoulders over Duel for total extra damage, but Duel gave far more damage against the bosses. Which isn't too surprising. Not too many enemies take six hits to kill, and the ones that will take a lot of hits to kill, tend to have have high defenses, making the +10 rather useful.
Personally, I tend to Max out Duel, than max out Heavyhand, than I worry about getting Massive Critical and Prevision Strikes to Level 2 (Level 2 due to the limited number of skill points).