Mature Games vs. Teen (Halo vs COD)

I have a problem... I'm not allowed to buy mature-rated games. For the singular reason that my parents believe it makes my little brothers more violent (they're 13 and 9.) We had Halo 2 for quite some time. My mom walked in one day when we were playing split screen, and threw a fit because my brother was yelling at me. We weren't allowed mature games from there on.

Contrary to that belief, we have COD2 & 3, which have very similar combat to Halo 2.  During multiplayer matches, there is wanton killing of realistic people, rather than Covenant Elites. Why, therefore, is Halo mature and COD not? The only difference I can see is that there is "blood" on Halo's box... (Where??? Not intrusive, anyway.) Anyway, I need to convince my parents otherwise, so we can buy Halo 3 when it comes out.

Can anyone give me some pointers?

Well here is what I think.

 The Call of Duty series is rated T because there is some educational things there that you would get in high school.

Halo has...well some mature themes in it.  Deep religious things with the Covenant, a torture scene in Halo 2, and blood (Call of Duty has no blood at all).

So...ya...just my opinion.