I have owned an original Nomad 883 since 2014, and built a Shapeoko XL this summer. I agree with the previous replies that the main question to ask yourself is “what do you want to make”. If what you want to make is 8" by 8" or smaller, then consider the Nomad.
The other question to ask yourself, “where am I going to put this CNC?” or “How much space do I have for a CNC?”. The Nomad is a great out-of-the-box experience. I would call it an in-door or home CNC. You can make something less than an hour after opening the box. I have mine sitting on a desk in my office, and it is easy to move. When Carbide 3D posts the physical size of the Nomad 3, that is the only area it will need.
The Shapeoko however needs more space, it is more of a garage CNC. It has an open design that makes it more customizable, but that also means more setup, more tuning, more mess, and (maybe) more maintenance. If you ever needed to pick it up and move it, you would need to go through the tuning process again. The smallest Shapeoko has a footprint of 28.4" x 23.9", but take a look at the Enclosure Zoo and you will see how much real-world space it usually takes.