I imagine Dark Channel is very useful for a Sentinel in Casual. Start bothering when you play on Insanity.
I agree with the other comment, get what you think is fun. If my Tech Armor is down, the next defensive ability will always be Tech Armor. Reave is a great choice too, but does your Tech Armor get depleted often? If you take reave it's for Health regen when Tech Armor is down, and Sentinel is all about Tech Armor. I don't have experience in casual but if you're most of the time fighting unprotected enemies you can take Shredder Ammo.
