vacuum air pump VS water vacuum pump

Vacuum pump has three model, water type, air type, oil-free type. which type is better for cnc router?

water vacuum pump, usually choose 5.5kw,7.5kw, need 3 phase voltage,it with a big water tank,  the advantage is cheaper price, but it rust easily.

Air vacuum pump is what razortek mainly use. 5.5kw double stage air vacuum pump, no need water, small volume.
it with air pressure valve, air filter, the silencer is optional. price is about 1.5times of the water vacuum pump



There are oil-free air vacuum pump, costly , similar like german BECKER pump, costly ,but performance is better.if there are enough budget, oil-free pump is good choice.
because it is small size.good qulity,