Dragonborn's story and the added area of Solstheim are rather good. I felt that the final battle in that DLC was a little lackluster, but that could be because I generally play the game on a low difficulty setting. Dawnguard adds the vampire lord perks and those can be really nice. Both DLCs have hints to Morrowind, Dragonborn (being the more obvious) is set on the island of Solstheim which is an island off of Skyrim and Morrowind and Dawnguard having the side quest once you enter the Soul Cairn to collect these pages that belong to Jiub. I personally think Dragonborn is worth more than Dawnguard because to me Dawnguard's story is kinda boring and the only character that makes Dawnguard worth-while is Serana. Also it seems like Dragonborn has a bit more story content than Dawnguard.