Hi! I like to say that TNG is the toolbox, and Voyages is what it can build if you spend 2-3 years and thousands of hours working with it. =) So if you want to create your own Galaxy from scratch then the TNG guidebook is good. If you want to jump straight into almost infinite adventures, then Voyages is for you. They both work well together also.
As far as rules go, Voyages contains a streamlined ruleset that I now vastly prefer. But the original ruleset in the guidebook is solid and works well too.
Hope that helps!