Saxons is a sandbox campaign, focusing on kingdom management, warfare and diplomacy (actually, the diplomacy system is unique to the expansions, as diplomacy is storyline/quest based in the vanilla campaign) but light on questing and with close to no storyline.
The vanilla campaign is heavily scripted with a branching storyline and includes "choose your adventure"-style text quests, with "roleplaying" elements prominent compared to the "wargame" stuff (as in the game title).
Both are definitely different (and great) experiences. I'd suggest to start with the main game, and play the expansions campaigns after that. I'd say the main campaign is the real, full experience, and the expansions are interesting alternate ways to play the game.