Ethics Reviews & Comments welcomes both types of contributions. Here, in a nutshell, is the difference between the two types of articles.
Reviews. Typically, reviews attempt to address the book as a whole and to give the reader an accurate and well-rounded view of what the author is trying to accomplish. A review will usually have a descriptive component (intended to provide information about the book to the reader of the review) and an evaluative compoent (intended to assess the strengths and weaknesses of the book in the reviewer's eyes).
Comments. Typically, comments focus on a narrower set of issues than a full review and tend to have a much more extensive evaluative component. They are not necessarily shorter than reviews, although this may sometimes be the case.
Who decides? Typically, authors are asked to categorize their own piece based on the guidelines given above. The Editor will then review that decision.