@Vandy Hmmm, I'll admit to not have the most extensive knowledge of the Halo universe (I know the basics if the story, and the motives that the Covenant had for attacking the humans) but I think the Reaper War does gives plenty of material to make a gritty war story, I mean on one hand you've got the impending sense of doom the reapers give, the husks (not to mention the effect that having to fight them has on soldiers), and you can always have some of an air of paranoia due to the threat of indoctrination.
But, that's just how I see things. If you think the Halo universe is better suited, so be it. It might take me a while to make a bio if you end up doing the Halo RP though (due to, like I said, my lack of extensive knowledge of the setting)