Warband is the stand alone expansion, it has better graphics, more quests (though the game is nonlinear and not quest based), a marriage system (allowing one to marry into a noble family for political leverage) and an additional faction and country for that faction, states Rolf. Be warned, one will feel overwhelmed and a little over one's head, but Mount&Blade is one of those awesome indie games that few notice, proclaims Rolf.
Warband is well worth it, suggests Rolf, however, both have long lists of mods found
here, states Rolf.