COD4 is awesome. Although it doesn't restrict the dead from speaking (just like the old Rainbow 6 on PC) due to respawn, it's sheer madness and fun altogether. However, it's practically all run and gun (even with the melee). HALO3 is a different experience. Although it's also a run and gun game, there are options to use vehicles (except for small maps), and yank-out turret based weapons (plasma cannons and gattling gun). Overall, host have full control of the game in terms of options (removing auto-aim and all) and weapons (removing laser pointer in COD4).
I haven't see new maps yet for COD4, but there new maps available for HALO3 but it costs 800 marketpoints (US$10) from XBL. But these said maps will be available for free by March or April. Advantage is the maps downloaded later is free. Disadvantage is that those who have paid for it knows the nooks and cracks all over the place.
In terms of gaming, refresh rate from hosts in US are faster for those who joined in the server (from Pinas or other). Pinas' hosts lag a bit but some actually have fast connection. Passing the ball as host from one user to another is easy. Trasher talkers can be muted or can be booted.