Reach has a good multiplayer, big and small playable, and forge-able, maps, ok campaign, better forge, firefight, a theatre were you can save full videos including campaign ones, cool Armour with attachables, and an awesome customs.
Halo 4 has a cool multiplayer, good campaign, bad forge, too small of maps to really do anything with, boring customs, awesome armor, including visors, Spartan Ops, but can only play with gold, and ordinance drops.
Reach has more advantages