Depends upon how much you want to know. The preview is much more in-depth (with its analysis of each player and the number of players included) than the guide. For example, the guide discusses 15 QBs while the preview discusses 27.
That little PFW booklet is not bad. Much more detailed per player and covers prep background, as well as college, and goes a little deeper into medical history. Player analysis is a layer or two deeper as well.