Operation: Anchorage and Dead Money both feature a linear path in which the player essentially starts with nothing at the beginning, and cannot be returned to after completion.
The Pitt and Honest Hearts both feature story driven open world exploration, and can be returned to after completion, but a faction may be missing afterwords (Pitt Raiders for the Pitt and White Legs OR Sorrows and Dead Horses for Honest Hearts)
Point Lookout and Old World Blues both feature open world exploration with fairly short main quests, but very expanded side missions and a lot of locations to explore.
From what we've been told, I think it's safe to draw the following conclusions:
Mothership Zeta and Lonesome Road both feature a linear path, but the player isn't stripped of gear and the add-on can be returned to after completion (albeit, most areas are closed off after completing Mothership Zeta)
Broken Steel and Gun Runners' Arsenal both feature a large expansion to the core game (Broken Steel adds more main quests, a few new weapons and clothing, and a small, new area while Gun Runners' Arsenal adds a bunch of new guns, mods, ammo, and challenges)
So my point is this: Which did you like more for each pair, the Fallout 3 DLC or the Fallout: New Vegas DLC?