MGS4 was a huge letdown for me...
MGS1 > anything halo
But Halo 3 > MGS4
Both have somewhat underwhelming campaigns...but halo 3 is much more fun online, or with 4 player co-op. It delivers in online and coop what it lacks in campaign "epicness".
As a halo 3 player, I am blowing up Scarabs with my buddies while you are watching a 45 minute cinematic where Otacon cries for the 3rd time...