Shields can block poison dots ticks.

The damage output between a 2H and 1H is very noticeable and does make a difference. Having both sets is fine.

You just can't switch gear while in combat.

Solo PvE/PvP = 2H. If you don't have a healer, your best bet is to kill the enemy faster than your health loss.

PvE with healer = 2H versus normal quest mobs.
PvE with healer = 1H + Shield versus Champions and Hero quest mobs.
PvP with healer = either is fine, but depends on objective and situation.