Marlene is the marksman/support (the mortar provides higher rate of fire and generally suppresses soft targets better, while the HMG is somewhat more useful at these distances too). Her frame is also the slowest (and now oldest) of the bunch.

Sarah is the close-up brawler (20mm autocannon that spams TONS of rounds per second), optical camo, and a 'stun'-function means she tends to infiltrate and then shreck their mechs.

Lang is our sniper/can-opener with limited versatility in combat (less payload).

The key difference between Lang and Marlene is that Lang has a command-FRAME that provides her with much better situational-awareness, and much better sensors than anyone else right now [i]until somebody apps a Zaruga[/i].