Since I want a good experience rather than a handful of "okay", "meh" and "ugh" experiences I'd go for an expansion of proper size.
Bite-sized DLC's always end up feeling like they lack things and a full fledged expansion can actually put polish and flesh on every aspect of the content.
That is if the developers of the expansion prioritize every aspect and try to flesh them all out accordingly.
Let's just look at the DLC's we had.
Operation Anchorage - A fight simulator with little to do but to run and gun, had this been an expansion we could probably have explored each of the areas in the simulation a lot more and been given more options for how to handle them rather than just a linear path of run and gun. We could have used tactical moves with soldiers, we could have used spy tactics, we could have used infiltration tactics. But no, it's just run and gun. The one DLC where we could have explored the war more in depth and all we get to do is run and gun.
The Pitt - An amazing concept of a city which had a lot of interesting aspects to it. The relationship between Ashur's "Raiders" and their customer base, the psychological terror that the slaves are going through. The transformation of Trogs and Wildmen and the emotional damage that it brings to anyone close to those that transform. The wildlife of this weird mutation. The lore around the city. Tons of things could have been explored and while some where it felt too short and was over before we knew it and Wildmen were shafted.
Broken Steel - Do I need to say it? Enclave path, why no Enclave path from the beginning of the DLC? When we sided with Eden or told Autumn to step down why was there no way to side with them? Why is the only way we can go against LB through nuking them at the end? We could have explored the consequences of 'all' the players actions a couple of 'months' from the final clash at PP instead of right after and seen how much of an effect our choices have had, instead we wake up immediately and are forced to help out the BOS even more even if we did everything we could to try to go against them before.
Point Lookout - The tribals, the smugglers, the locals that are not mutants, the hillfolk, the pre-war lore, tons of things that could have been explored upon more in depth to give us a completely different feel from the CW. While the DLC's main storyline is paced well and is somewhat interesting there are a lot of things in there that just feel lacking, like they're there without any proper explanation.
(I might list the rest later, I'm too tired to list them all right away)
Point is, each of the DLC's had they been proper expansion size could be amazing. But when you have a finite time to work with and you have to jump off a project and move on to another one 4 times over then a lot of things have to be skimped down. An expansion allows more time to work on every aspect of the new content so for me it's far better.