Which stat is better: Increased Energy Shield or Maximum Energy Shield?

I am trying to increase my survivability on my ED Contagion Trickster build and am wondering which stat is better, Increased Energy Shield or Maximum Energy Shield? I'm sure it depends but any explanation would be appreciated.

As you say, it REALLY depends.
Do the math, and see which one gives you more.

In general, the more you have of one, the better the other is.
If you have a ton of +flat ES, then +%ES becomes more valuable.
If you have a ton of +%ES then +flat ES becomes more valuable.