A number of similar of elements in both bands, but the results are different to my ears. Agalloch usually maintain a consistently doomy, melancholic feel, whereas Opeth has a number of different moods in their music. They can sound progressive, aggressive, or downright sinister. They also often sound a lot more "jammy" and less orchestrated than Agalloch. I couldn't really choose between the two, but Pale Folklore is one of my all-time favorite albums.