Who should I start as a flex?

The general rule is to start the RB if you have a choice. I think Benson is the less risky option because he will definitely get the ball, and Detroit is suspect against the run. However, though Stallworth may be more risky (given all the options at Brady's disposal), he also has a higher ceiling (Pats offense is ridiculous, and they will p, p, p, even if they are up by 30). So, if you feel confident against your opponent, then play Benson. If you feel you might need something extra, then take the risk and go with Stallworth.