Is it ever better to get AR/MR before HP?
Wouldn't it be better for someone like Diana to get a Negatron Cloak before, say, a Giant's Belt, since she has her shields? Or is that another stat-confusion trap?
Wouldn't it be better for someone like Diana to get a Negatron Cloak before, say, a Giant's Belt, since she has her shields? Or is that another stat-confusion trap?

It's almost always better to buy resistances and potions if you're behind in lane than to buy HP, especially if you have a shield or sustain. After a single resistance item, though, you're definitely better off getting health.
You're also better off buying health if the enemy has the damage to 100-0 you.