what --->I<--- think...
call of duty 4 will be fun for a good long time.
halo 3 is fun and if you have lots a friends you can spend all day playing custom games or goofin around in social slayer.
you will prolly play it for a very very very long time.
CoD4 addictive and more competitive and custom games arent as good as halo though.
CoD4 more tactics
So if you dont have many friends on xbox live your best bet would be CoD4
if you have a ton of friends that play halo 3..buy halo 3