"could" is a little bit more polite, though this form is not as significant in English as it is in German. The "please" is the most important part and will make much more of a difference than "could" ever will. I imagine that is true in practically every language.

Royal Shiny, In a formal situation, or if you are asking help from someone that you don't know, use "could". It is less direct and more polite. In an urgent situation, or if you are speaking to someone you know well, then the less formal 'can' is acceptable. It is more direct and less polite. As fdmaxey inferred, try to use "please" when making a request...and don't forget "thank you" afterward. One of the important functions of the modal verb 'could" is to express a polite request. Since verbs do not have polite forms in English, as they do in German, French, Russian, and many other languages, 'could' is a valuable 'tool' in the native English speaker's vocabulary. A non-native speaker, who may not have a complete understanding of the English speaking culture, is wise to be polite. It also makes a good impression...something that many native English speakers seem to have forgotten.

"Could" (in the above context) = "Would you be able to... ?" Which is to say: "Would you be in a position to... ?". Comparable to the German "Könnten Sie bitte langsamer sprechen?" ("Could you speak more slowly, please?") "Can" is just that: "Are you able to... ?". So, "could" is definitely more polite; but, indeed, the practical difference is negligible. In reality, English doesn't have a true Subjunctive (except in a few remnant cases, like: "If I were ...," and such).

I think, the modals could is better to use as its more nice at the pragmatic.

both are correct,indeed.