From a usage standpoint, here is how these words are commonly implemented: Needless to say, my advice did her no good. It is completely unnecessary to keep repeating that. 'Needless' is a lot less used than 'Unnecessary' so I wouldn't worry about that one unless you're starting a phrase like I did above. Saludos!

http://forum.wordreference.com/threads/needless-vs-unnecessary.3071257/ hope it helps

Technically there is not much of a difference in meaning But they are used differently in sentences. I would go with the second persons answer on this question.