Ideally, I prefer a moving gantry for no other reason than you get the largest envelope for your footprint.

I would however only consider it for an aluminum or steel built machine. You gain a tremendous amount of rigidity with a heavy fixed gantry.

As for the weight(table vs moving gantry)- I'd reckon your gantry + Y&Z axis components + the spindle and tool plates will outweigh most torsion box tables + a part.

Aside from weight, the moving gantry is going to experience much higher inertial loads if you consider the leverage at the tool whereas the table will have a very low center of gravity.

Just my 2cents....