You should balance two things.
- The attack difference of a 1 handed weapon and a 2 handed weapon
- The defensive stat of the shield
These days the main role of the shield is to give elemental and physical percentage protection. For example, a ornate shield can be imbued with 15% of fire protection and it'll give a 5% of physical protection. When you're facing 8 monsters (boxing) this protection can be very helpful if the monsters deal more damage than you can heal. It is also very useful when killing bosses as they have very strong spells. Also there are some shields that give offensive stats (shield of corruption, ectoplasm shield).
You see an ornate shield (Def:36, protection physical +5%).
Imbuements: (Powerful Dragon Hide 16:52h).
However the extra attack from a 2-handed weapon will benefit your life and mana leach. You'll kill faster and improve your exp rate. I would say that the extra attack give you more damage than the extra skill you can get from shield of corruption or ectoplasm shield in most of the cases.
To sum up. Prefer 2-handed weapons if defense is not a problem and you want to maximize your exp/h while hunting. If you want to have a more shield hunt or need extra protection to survive prefer a shield with % defenses.