Having run both types, I think both have advantages and disadvantages. For the move able column, you can have an almost unlimited bed length. I programmed Cincinnati routers at Boeing that had 420 inch long beds. The entire column moved on rails. One disadvantage of this construction method was a limited amount of tools. Because the column moved, the tool changer was carried on the column. So our particular machines had 12 tools. There is also a requirement of more floor space, since there was a cable track on a "flying bridge", to plumb air to the moving column. The machine gantry itself was less rigid, since it had to move. One advantage of moving the columns instead of the bed is load capacity. We could handle fixtures that weighed several tons, with no issues. Since the load itself is "static", the machine does not have to accelerate or decelerate the load. We had a system where we could be loading or unloading parts and Fixtures while the machine column was Cutting on another section of the bed. Our fixtures were all vacuum based, and it was easier to plumb the vacuum lines on a fixed bed. The longer your potential parts are, the more a move able gantry makes sense. Imagine having a 250 inch long part. You'd have to have 500 inch long ball screws to move the table below the spindle. At certain point, it just becomes impractical to move the bed. For smaller parts, a move able bed gives you more rigidity and allows for more aggressive metal removal. It also takes, in general, less floor space, since the bed length limits the part size.

That made no sense to me whatsoever.