LI think your guys have access to a relic that essentially gives a character lieutenant re-rolls, so you might want to avoid the lieutenant unless there was a specific relic you wanted to take with you. A captain is a good option, especially if you have a decent amount of CP in your list so you can give him an aura of re rolling any hit roll if you need to. 

Personally though, at that price I like the idea of a Phobos librarian, but I’m always massively high on that unit. As a fist, you get so many tools with him. You get psychic powers that buff his strength and toughness, heal himself or other multi wound models, and do orbital bombardments (not to mention getting in a good position to smite and deny psychic powers). You also get a relic to turn his pistol into a character killer, or to make him into a lieutenant, as well as all the defensive relics that are available. He also has a power that makes him harder to charge, so if you want to take an eliminator unit with a Sarge with the sniper bolt rifle (maybe for the special issue bolts), you might be able help protect them from charges with the right positioning.