I feel that Reach had better objective by far since everyone was assigned one AA and had to work together to gain power weapons on the maps. Reach also had better customs than the repetitive fat kid/drive or die maps in halo 3 and the lack of custom options in halo 4. Also forge world allowed a lot of creativity.
But I feel that halo 4 had the better and interesting campaign. I also like slayer in halo 4 more than reach.