which is the beneficial keywords for promotional point of view long tail keywords or short tail keywords ?

Long-tail keywords are  three words or more, and they are more of a phrase or question than a broad description. The answer depends a lot on your market and the competition of your keywords. You can find out how competitive your keywords are using the Google AdWords Keyword Tool.

Long tail keyword are also known as Keyword phrase.
Long tail keywords are easy to target & achieve high ranking because they are low competitive.

Long tail keywords are beneficial for promotion.

Long tail keywords are used when the website wants to refine search terms to the web page, as well as when the searcher is looking for something rather specific.