Warband is basically an upgraded Mount and Blade... because, well, it is. But that doesn't mean its bad! Hell no! It's awesome! But when I say upgraded, it is literally Mount and Blade's map with new areas, factions, things to do, units etc. So if you are buying Mount and Blade, get Warband...
or....
Muskets? I love F&S just because it makes sieges, multiplayer and battle lines so much more fun with muskets. Believe me, play a siege battle with 100 other players and you will get shot soooo many times running to the walls, only to scale the walls, slaughter the defenders and open the gates, letting your cavalry charge in. Then, when your soldiers are chopping the enemy up, the marksmen take the walls and turn the walls against the defenders. There is nothing more awesome than shooting through a slit in the wall as the enemy rushes your castle, and then fighting him off the walls with Fire and Sword!
Although, always join a server with the ladders that can be put up and kicked down, gates that can be opened and doors around the back of the castle. Believe me, F&S multiplayer is still kicking!
EDIT:
Also, muskets aren't really too much of a problem, hell, arrows are sometimes more of a problem. See, once a musket fires, it takes AGES to reload. In a battle on a flat-ish area, each side usually only gets one volly of musket rounds at any type of cavalry, and then once the muskets are getting chopped up, the infantry follow in, mopping everything up. The only time muskets get really annoying is in sieges, but in a REALLY long siege sometimes you can wait for their ammo to run out. The average amount of shots you will get is about 15, because that is the starting ammo and buying ammo costs a LOT. But, luckily, ammo refills after every battle, so its like buying an ammo pouch, because its how many bullets you can hold, not how many bullets you have.