I picked Jack for the biotic shield without a second thought. She was talked up as possibly the most powerful human biotic in the galaxy, and that's the only reason Shepard recruited her. It would have seemed like a waste if she never got to do what she was recruited to do. Samara was also a logical choice, but I had her escort Chakwas, Kelly, et al back to the Normandy. I'd rather trust them with a Justicar than with a mercenary.