I've recently had the opportunity to review Warband: http://www.game-debate.com/articles/...433&game=Mount and Blade: Warband and have enjoyed it immensely.
Single player is a fresh-feeling evolution of M&B with added features such as a new map, a new faction, morale, and marriage.
Multiplayer is evenly balanced gamewise and alot of fun. Combat techniques will take alot of work and polishing tho' (some people have already become very proficient via Beta, and are very hard to beat!) The usual frustration of multiplayer combat games: lack of player discipline, total lack of basic tactics etc is compounded by no in-game VOIP. But it's a great place for a brawl.
Mount and Blade or Warband? Well, obviously there is a big price difference Warband is still new and the price will likely stay close to retail for a while. M&B can now be found at incredibly good value....And is your passport into literally hundreds of different worlds due to the vast array of fantastic free mods available via the Taleworlds site http://forums.taleworlds.com/index.php/board,63.0.html
Get them both! They are both extraordinary games in their own ways!