Neither adds a really significant chunk of area to each respective game, but they manage to make the most of it with the quests and exploration.
Both require you to help one side or the other as they both vie for power
Spoiler
Almalexia vs Helseth in Tribunal; Dawnguard vs Vampires in Dawnguard
And lastly, both requires you to travel to an alternate dimension of sorts
Spoiler
Clockwork City in Tribunal and Soul Cairn in Dawnguard