In a structured job interview all the candidates are asked the same set of questions in the same order. This ensures that they are assessed on the same parameters, translating to a fair and objective assessment.
The flipside to this style of interviewing is that they may not elicit much information from the candidates.
Unstructured interviews on the other hand are conversations that follow promising lines of inquiry as they appear. You get to know more a lot more about the candidate this way, but it makes comparing the candidates difficult.
Which is the best bath? Depending upon the requirement it could be a either or neither. In my opinion the best path would probably the middle path. No i am not referring to the path showed by the Great Gautam Buddh.
What i mean is, stay flexible in your line of questioning – but have a core set of questions common for all the interviewees. By preparing these core questions in advance – you can be assured that all key points are covered. While, the unstructured element of the interview opens the door to productive areas of enquiry which you may not have anticipated.