Have you done any fiddling with cardboard cutouts to see which configuration gives the most travel in the least floor space? They are comparable. If you have a fixed gantry and a moving bed the bed travel has to be slightly greater than the bed length in order to go past the centerline of the tool in both directions so this means you need floor space slightly longer than twice the bed length. A moving gantry also has to travel slightly further than the table length to get the tool centerline past the ends of the table. It might seem that the moving gantry machine needs only a bit more than one table length but the rails for the gantry have to extend past the table a significant distance so the total length will be around one and a half the table length.