The key issue is which commentary answers the kinds of questions you are likely to ask. As usual, you should take time to look at what's available. Compare before you buy. One consideration for comparison is what theological perspective the author is writing from. Another consideration is the date of publication-some historical and cultural information in older Commentaries might be outdated or incorrect, but the devotional material in them would be timeless.
Commentaries come in many different sizes.
You can get a Commentary or set on the whole Bible or for individual books of the Bible. Using a Commentary Devotional Commentaries, such as Matthew Henry's Commentary , focus on the spiritual significance of the text for our lives. Expository Commentaries focus on explaining the text. Exegetical or Critical Commentaries focus primarily on technical issues related to the Greek or Hebrew text and its interpretation and are written primarily for specialists. Click here to view a sample bible commentary page.