Chris is hard/normal and Jill is easy mode. Jill gets a new weapon and several puzzles are optional. Chris is the real experience.

The only tip I'll give is that the first Bravo team member getting chomped on can net a whopping 30 rounds if you go search him immediately. If you don't the zombie eats the whole body, and presumably destroys the rounds. An extra 30 roudns really helps.