Yeah, I think that you should sell the 360. I'm guessing that you probably bought the 360 around the time Halo 3 came out like a lot of people becuase you didn't mention any other games.
You should never really buy a game console just for 1 or 2 games, but if you aren't playing the Xbox 360 hardly anymore then sell it. The majority of games are 3rd party games anyways, and the 360 doesn't really have many other really good exclusive games. Also, the majority of the 360's "exclusive" games come out on the PC as well. With the PS3, at least the exclusive games are guaranteed to be exclusive to only to the PS3.