I don't know how final the stats for UNSC ships are, but here are some thoughts I have regarding Marathon and Halcyon cruisers. I was thinking that Halcyon class cruisers should have a more substantial amount of hull and armor points, should be more expensive, maneuver slower and it should have weaker weapons. While, Marathon class cruisers should have a lesser amount of hull and armor points and should be cheaper to build (compared to the Halcyon), be relatively more maneuverable and have more powerful weapons.
Again, I don't how final stats for UNSC ships are, but I'm hoping my suggestions would help the Marathon and Halcyon cruisers fit with cannon better (and match their descriptions better). It was my understanding the Halcyon cruisers were crazy strong, but expensive and slow (before being refit, Halcyons had outmoded reactors) and Marathon cruisers had lighter armor, were more maneuverable and were cheaper to build, but had better weapons.