My personal take:
Use the retweet button if you want to retweet an unmodified version of the original tweet. This gives full credit to the original poster, as to your followers it will appear with his profile pic and Twitter handle.

However, in case you need to add anything, the old fashioned RT is your friend. This addition can be an endorsement, a factual addition, a correction or anything else. Another use of the old fashioned RT would be: if you want one or more of your followers to make a special note of the original tweet by including the @mentions of their Twitter handles.

See question on Quora