If you're shield and sword then choose Sten. Alistair already starts down the same tree you are going down. --- -No getting off the hook for free today, ladies, I'm not through playing. Besides, this time you might even give me a little sport- ~Albel~
Alistair is funnier and has better dialogue than Sten. Plus, I feel that he is better than Sten for being a tank and dealing damage, as long as you level his attributes and skills correctly... --- "I feel sorry for people who don't drink, because when they wake up, that's as good as they're gonna feel all day." GT: JRBMaximus
Sten is a damage MACHINE. If you give him the right combination of talents, skills and tactics, Alistair will never be able to beat him for raw damage-dealing potential.
However, Sten doesn't get the armor (and sometimes magical) bonus from carrying a shield, and there are a very limited number of truly AWESOME two-handed weapons in the game. Good one-handed weapons seem to be much more common (of course, the top-tier two-handed weapons that DO exist are pretty darned amazing). When it comes to tanking and soaking damage, a well-leveled Alistair will beat a well-leveled Sten every time.
Personally, I picked Alistair because he is a lot more interesting as a character, and has amusing random dialogue with nearly every other party member. Plus if you give him Shield Wall, he is as close to invincible as this game will let a character be. With a healer like Wynne in the party, he can tank enemies like no other, and his damage is nothing to sneeze at.
Al's dialogue is very funny, Sten is just bleh, so the choice is beat the game or beat the game and have a good laugh too. --- I'm sorry, I can only find his head, I'll call you back, when I find the rest of him.