Dragonborn, you get to have a whole new island to explore, new ruins, dungeons, towns, people,and a lot of items to posses.
Also I know the main quest is short, but there is tons of side quests to have, besides, you get a lot of new creatures to fight.
While Dawnguard is cool too, you're still in Skyrim after all, and there is barley any side quests, and you know, killing Vampire/Dawnguards all the time gets boring after a while.
I spent 60 hours of game play in Solstheim, I really took my time, I explored everything and finished everything, while I tried to do everything possible in Dawnguard, It only took me 10 hours before I realized that there is nothing left to do anymore.