okay while Marcus doesn't have energy sheilds before anyone brings it up, Marcus has way better weapons and he has way more experiance fighting things very well like Master Chief in a way. Locust are strong, can take a lot, and he has taken down Brumak, Beserkers(WAY stronger than MC), and a lot of other things. The weapons Marcus has at his disposal are meant for taking down things like that too, imo, this doesn't even need to be discussed!
Chief has alot of advantages considering he has armor and what-not. But Fenix has alot more experience, and seems to be more battle hardened. And Fenix has experience taking down enemies that have advantages.
I'm gonna have to say Marcus. Chief is cool and what not, but c'mon Marcus is tough. The enemies he fights are insane, and he also has the better weapons.
About MC's armor. As far as I'm concerned if a little bullet can penetrate chief's armor a chainsaw sure as hell can.