Skill is arbitrary
Story is personal preference (unless you have actual points to make about the structure of the story, elements, pace, etc)
Community can be discussed, but only factually.
I think that if you wanted to compare the two you should start at the base differences:
Playstyle
Gameplay
Players
Campaign
Compare the two, rather than picking a victor. There is nothing to gain in writing something you believe is true (as it is your opinion on display, unless of course that is the point of the paper is to make an opinion on something).
Start with basic comparative elements, and then give a miscellaneous category for features that cannot be defined.
Begin with explaning the backstory for the two games (simple overview of history/sales history/that both are highly rated, regarded, etc), then get into specific details surrounding the two and why they are different/the same.
Without knowing the exact assignment I cannot give the best advice, but if I were you I would tackle this in the above manner.
Explain your preference, but only after the review, and don't feel bad making halo look good, just don't make it so obvious.