Both, but the story was sooooooooooooooooooo good for Dawnguard. Dragonborn's story was kind of crappy, but the exploration and missions were great. I would say Dawnguard.
Dawn guard has a much longer story. And you get a crossbow. (Should be in the game already) but dragon born will probably give you more gameplay because it gives you an entirely different island too explore.
Dawnguard is good... at first; then the horrible truth dawns on you that if you don't complete it straight away characters like shopkeepers have the potential to be brutally killed in a random vampire attack.
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.
dawn guard is a pretty awesome quest line, but dragon born opened up a whole new are to frolic in. also brought back some memorys of morrowind, specially that stilt strider.
I liked Dragonborn more. Dawnguard was good, but for some reason I felt like main quest sort of dragged on and there were points where I wasn't as interested.