The up side of a fixed gantry, moving table means, for smaller work pieces, smaller drive mechanisms as (generally) you're not throwing so much weight around and a more rigid structure. The downside of a fixed gantry is a larger footprint for the same work area. A 500mm long area needs the table to move 1m, whereas with a moving gantry it traverses 500mm plus the 1/2 the gantry depth in the X-direction (often the bearing spacing) so potentially as little as 600mm.