This really depends on what type of end product result you are looking for. The milling machine is capable of creating things in a wide variety of materials and is capable of producing professional looking results. 3D printing is great for complex shapes or prototypes, but the materials are limited to specific plastics that can be extruded (commonly ABS and PLA).
The best part about your Zen Toolworks machine however is that it is capable of both. The machine platform we use for milling is the same used for 3D printing. To switch between the two would require a slight electronics change and switching from your rotary cutting head to a 3D printing extruder.
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