See if this website will help:
http://www.wittcom.com/communication_tips.htm
Both. Since both are existing in communication. If you are communicating through mails isn't it that you used the written communication? It should be clear and well constructed. Same to verbal, you should speak loudly and clearly.. for better communication.
Command on language with clear thought, knowing exactly what to communicate.
Body language is also important in verbal communication.
if you are speaking to a mature listener i believe verbal, but when you are speaking to an immature and ignorant person, i believe written, so they will have noone to interrupt and be able to let finish!
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them