3D printing is one of the next and biggest trends in technology. CNC machinery is also huge, especially to makers such as myself. This is why I decided to create a short, but comprehensive article comparing both of them.
3D printers are perfect if you have a precision based printer and want something to come out extremely durable. People are doing things such as 3D printing houses and furniture nowadays. I, myself have been going to a maker-space to try 3D printing enclosures and CAD models. I also recently bought an extremely low end and basic 3D printer for home called the Hero Delta 101. That printer will be used to print enclosures for micro-drones.
That being said, the main problem I have with 3D printing is speed. The speed of the 3D printer is extremely low and most of the time, it is better to just make molds and fill them with melted plastic and silicon. This is especially true if you need to save time.
Now CNC machines are much faster. You can use a CNC to print really beautiful cases for cluster computers or mining or the Raspberry Pi all done in acrylic. You can also etch stuff in CNC. The precision quality is seemingly much better. The downside though is that you can't print 3D models or create things as heavy duty as a 3D printer. The upside is quality and time in comparison to average costs.
Basically when you look at the pros and cons, it really depends on what you want to do. If you want to make box style cases and don't need heavy duty molds, go with a CNC cutter. However if you are making an enclosure that needs to be more sturdy but would take much more time, it is often best to use a 3D printer. That is why I like both depending on what I want to do. 3D printers and CNC machines have been proven to be useful tools for me from time to time.