This really struck me when out of the blue, I suddenly realized i was on the last quest of the Dragonborn storyline. It happened really abruptly and suddenly, and I found it weird. I hadn't even been around the whole island yet! I would have thought that it would have taken me all over. It's not even that it's short, but it only takes place in 2 world spaces, Solsthiem and Apocrypha. Beyond that, the DLC does a very poor job of introducing side quests to you as you progress through the main quest, so you kind of do what you're told to do if you're a roleplayer, so your at the final quest before you know it. It was also sloppily laid out, especially in terms of Black Book placement. Like, I stumbled on to one before I had even begun the Main Quest, and it kind of messed up synchronization with the main quest in terms of dialogue. Furthermore, Mirraak....you see far too little of the title character. Granted I haven't beaten the last quest yet, I just got it, but up until now I've only run into him once. It just seems very poorly paced.
In Dawnguard, it feels like a fairly excellent and long adventure, despite the lack of optional content. You travel to a bunch of new Dungeons across Skyrim, venture into the Soul Cairn, and travel through the Forgotten Vale. And it's all very cinematic in a sense. And some of the characters are really interesting, such as Serana, or the last Snow Elves. The Vampires feel like a legitimate threat because as I progress, I get attacked by them more and more and people around Skyrim remark on it. It just felt like far ore of an epic quest, that went beyond just fighting Vampires. Dragonborn kind of doesn't, while the Apocrypha segments are phenomenal, it's Main Quest seems to be lacking in scope significantly.
I will say that Dragonborn overall is the better DLC, but as for Main Quest? I think Dawnguard easily has this.