The difference is in accuracy and linear-motion complexity.
A lathe is less accurate as the item gets thicker. 1-degree of rotation might have 1000 steps, but if your cutting area is 10" away from the center, 1-degree is now 2", as opposed to being 0.1" when the blade is 1" away from the core of the spinning object. In addition, to cut a flat-side, on a rotary device, the CNC has to go through a ARC to create a flat cut while the object rotates. With linear steps, this produces lines in the surface that a normal mill would produce completely smooth.
As opposed to a normal table-mill...
A table mill makes curved surfaces with noticable track-lines, but perfectly smooth and flat surfaces and smooth individual angled-lines. The limitation is normally top-down cutting limited, or plunge-milling, which would require a fourth axis to do side-cuts. (EG, you need one less axis on a rotary/lathe setup.) You have linear accuracy, across the entire work surface, equivalent to pixels or 3D voxels, and better angular interpolation between those points, as opposed to the dice-cutting angular interpolation that will be unavoidable on a rotary/lathe style CNC.
Which is better? (The right too for the right job)
Depends what you want to make? The lathe is better for making tall statues with less complex hardware in a smaller space. The mill table is better for wide flat items which have more directional detail like wall-mounted art or floor art.
Ultimately, for statues, you want a nice 6-axis machine robot. The tool arm would have four, and the table would have two. The table rotating, and moving in and out, while the arm has the ability to plunge-y(ang), angle-x angle-z and raise-z.  |