Honestly, I don't mind playing COD if I can't play Halo for some reason. It's not a bad game, but as far as "realism" goes, those of us with military experience laugh at it. A lot.
Let's chat about the realism. First, it doesn't take half a clip to bring a guy with just a flak jacket on. You get him in the arms or legs, he will hit the ground. You shoot someone in the foot, they will not be able to sprint after you and knife you. So, as far as it being more realistic then Halo goes, not really. Second, their attempts at balancing the game kind of throws off the realism even more. They need to put more research into 5.56 and 7.62 rounds before they equate the damage done with an M16 and an AK-47.
Let's talk about realism with Halo. As far as weapons go, it seems fairly unrealistic. In real life, a bullet has travel time, which means you usually have to lead your opponent by a bit if they are moving. However, remember that this is a few hundred years into the future. So, ballistics in the future will probably travel a whole lot faster than the ones used in present times. Well, that makes the weapons seem more realistic, let's talk about damage. If a Spartan get's hit, they have about a ton of armor protecting their already hardened bodies, and it's full body armor WITH shields. So, the damage is fairly realistic. Now let's talk about the tech. It's not like it's flying cars, or robots that take care of your every whim. It's simply human technology with advancements made to them. Stuff that we are actually currently studying, in fact. Then the covenant. Honestly, fighting them is a whole lot more realistic than a random war with Russia, or some random terrorists who for some reason take a whole lot of ammo before they finally kick the bucket. The covenant are an unknown alien threat. For all we know, at this very moment, there could be advanced alien civilizations out there. So it's not unrealistic.
So realism (oddly enough) goes to Halo.
Now, let's talk about the STORY The COD storyline is pretty much all over the place. It's kind of difficult to follow, between the fps cinematics and the constant bombardment of enemies every level. To me, the story in COD feels like something out of a Tom Clancy novel rather than an actual, thought out, fact checked story that could get printed in a series of New York Times bestsellers. The Halo storyline is one that is adored by fans of all ages. You don't even have to pick up one of the books to realize that it's well thought out. It has all of the qualities of an awesome story (IMO). It has the hero (117), the antagonist (covenant), betrayal (covenant vs elites), odd alliances (elites/humans), and the sacrifice made to save humanity (deaths of favorite characters). It didn't have random plot twists, it wasn't gimmicky, and it was well thought out. It is unique. And every single Halo game has differences from it's predecessor that make it seem like a whole new game, but still Halo.
This is all imo, with a bit of fact here and there, but it's the reasons I prefer the Halo series over COD.