I'm torn between the Hearth Harvest (which has 3 DR bypass) and the Exceptional Hatchet (which i could put 30% damage onto). Which would do more damage?

Heart Harvest is better, as you can make it latter on also be exceptional addition to its other bonuses

Heart Harvest is better, as you can make it latter on also be exceptional addition to its other bonuses

 

So if i had Heart Harvest 

• Burning Lash: +25% Burn Damage
• Rending: 3 DR bypass
• Exceptional: 30% dmg

 

Or Exceptional Hatchet 

• Burning Lash: +25% Burn Damage
• 30% dmg
• Exceptional: 30% dmg

 

Which would do more damage?

You can have either exceptional or damaging 2 - you can't get both, they're mutually exclusive. So, with the parity in enchantments, hearth harvest is always the better damage dealer.