I'm not a fan of the Halo series but it's silliness never bothered me. In fact Halo's whimsy is probably apart of what makes the game so appealing to some people. It's a decently designed game with smooth controls and inoffensive game play. Granted, you are still blowing people to hell but the enemies personalities strikes a balance between lightheartedness and violent action.
I'm not sure why everyone is arguing about which weapons are more badass and realistic when it comes to Halo and Gears of War. I don't think Halo was ever going for realism nor was Gears. Halo is a fast paces shooter and it would seem to me, that the weapons were designed with this in mind. The Halo weapons fit it's gameplay, they assist you in mastering the run and gun strategy. You can run through a Halo level, rarely breaking pace. Emptying clips, swapping weapons and along with the shiny colors, you are almost hypnotized as you fall into "the zone".
The Gears Weapons seem to have been designed around a gritty visceral approach. Every weapon in Gears makes you feel like a certified badass as you dispatch your enemies. These weapons, like Halo also seem to have been designed around it's specific cover and shoot strategy of game play. There really is only one or two weapons in the Gears Arsenal that would transfer well to a run and gun game. The weapons in Gears seem more realistic but the weapons in Halo are more futuristic, fitting their respective atmosphere.
As for why Halo is so goddamned popular...,well, i can't comment on that. I have no idea. So, i can't argue why Halo might be a better game than Halo. Halo feels like EVERY other FPS that i have ever played, only, i've played better. I thought Halo 3 had a lot of epic and memorable moments but right as i was about to like the game it ended. Right as the game was about to start doing something different, it ended. When you people argue that Halo 3 is better than Gears because of it's multiplayer, make sure that you specify that you're arguing the multiplayer aspects because it is very important to know that Gears primary focus is the single player campaign.