I have the choice of both, I have been running a fixed gantry with a 700mm x 600mm table now for a few years with no problems whatsoever, and with a 2.2kw W/Cooled spindle capable of taking large cuts in wood, metal and stone and even fine engraving on glass, If I were to design a new machine I would decide on the maximum size of bed I want to work with and add 150mm for clamping and you need the rails as the bed of the machine and twice the length of the table and the blocks fixed to the table.
The moving gantry machine is a 7.5kw spindle 2.2m x 1.2m is on rack and pinion and has had problems racking, losing steps, also an inconvenience is the depth of sides when loading you can only load from the ends.
If you have the space to use a large machine I would go for fixed gantry over moving gantry everytime so much more ridged.

Phill