They both have their merits. Dawnguard has the better story imo, buffs vampires and werewolves, introduces a few new locations, and has uniques "toys". Dragonborn has more side quests, introduces a larger new area to explore, gives unique powers and shouts, and has some unique "toys".
Both are good, can't really go wrong with either. Dragonborn seems to be more popular and imo gives the most amount of new content for the price.
My favorite of the two is definitely Dawnguard, but it probably has a lot to do with the fact I wasn't a big fan of Morrowind aesthetically and Dragonborn brings back too much of that feel for me which is a turn off. I also liked Dawnguard's loot better and the Vampire Lord form and improved werewolves were fun. I did like a few things from Dragonborn, but for the most part I don't really care for much of the stuff I got from it.
"The madman is not the man who has lost his reason. The madman is the man who has lost everything except his reason." - G.K. Chesterton