How to not suck as a Sorcerer?

[ad_hoc]

At level 3, there's not much you can do. Mage Armor is better than Shield, because +3AC for 8 hours is more important than +5AC for one round.
But with Quicken or Twin at later levels, you will always be able to do something great when you need to.

Shield is better.

Mage Armour might not do anything for you and so will be a waste of a spell slot often enough to be significant.

Shield will almost always work so will be worth the spell slot. It is +5 AC versus +3 AC so it has more potential to save you. Finally, it lasts until your turn so it might stop multiple attacks.

Mage Armour is only good when 1st level slots are plentiful.

*edit* It is even worse with the Strength of the Grave ability.