Moving table/ fixed bridge designs are generally more rigid. It's easier to build a table that won't flex than a moving gantry that's rigid. But they take a lot more floorspace, which is why the moving gantry designs are more popular.
As any component gets larger in a linear dimension, it needs to be built more heavily. So you can build a smaller machine a lot more cheaply than a larger one. If you already have the materials, you need to decide whether you want a smaller, tighter build or a larger looser one. The further apart the X axis trucks are spaced, the more susceptible it will be to racking, if driven from the middle with a single screw. People wanting to avoid that use dual motors on the long axis, and either dual screws or racks and pinions.