what's the best for asking someone to write some word or sentence down? Is "적어주세요" or "써주세요"?
if theres another one better, please let me know

Question

what's the best for asking someone to write some word or sentence down? Is "적어주세요" or  "써주세요"?
if theres another one better, please let me know

Answer

Both can work fine for writing something down for you so you can understand better.
써 주세요 is a bit more general, so it might work better.  적어 주다 emphasize giving someone a note or reminder by writing it down.  써 주다 seems to emphasize the act of writing itself, although this is a very small distinction.
You can also say 글로 쓰면 어떻게 돼요? = What does it look like in writing?

Answer

편지를  써주세요(0)/  편지를 적어주세요(?)
글씨를 써주세요(0)/  글씨를 적어주세요(?)

선생님이 불러주신 내용을 받아 적었다.(감정 X)(메모하려는 의지)
오늘도 일기를 쓴다.(감정 O))
전화번호를 받아 적었다(감정은 없고 메모해서 기억하려는 의지가 있을 때)