If you can afford it do both! Each one does less than 100% for the smoke (which is vaporized metal....nasty, abrasive and conductive) and dust (larger particles of metal). The water table quinches the sparks and smoking drop outs, but does not kill the smoke under the material (between the material and the water) A downdraft system will remove most of the smoke but not stop the sparks. I finally gave up and built a containment room from heavy gauge plastic and used an exhaust fan to the outside AND kept the water tray. I have still had to replace three keyboards and mount the computer in a box with filters.
TOM CAUDLE
www.CandCNC.com