People have written *volumes* about just this sort of thing :) They're both okay, but changing "will" to "would" sounds more polite and friendlier. "Please" is *always* nice. Other distinctions are prescriptivist grammar, and you just want a pen, right? :)

When you ask "could you pass me the pen please" you're actually asking the person if they are capable of actually passing the pen. When you ask "will you pass me the pen " you are asking whether they would like to pass the pen to you. It's a matter of personal choice.

The second one is more polite and better way of asking for something.