Commentaries normally teach a little closer to what we call Biblical Theology (or O.T./N.T. Theo), differing slightly from Systematic Theology. Of course, there primary purpose is to interpret the text, et. al.

ST answers the question of "What does the whole Bible teach about any given topic.?" ST involves collecting and understanding all the relevant passages in the Bible on various topics and then summarizing their teachings. This differs from BT which organizes its topics historically and in the order the topics are presented in the Bible. Both are beneficial as are historical theology (a historical study of how Christians in different periods have understood various theological topics) or philosophical theology (studying theological topics largely without use of the Bible, but using the tools and methods of philosophical reasoning and what can be known about God from observing the universe). (Grudem, Wayne A. Systematic Theology).

As far as commentaries: Baker NT, Hendriksen & Kistemaker is the best--for the time being.

Reformed and Always Reforming,