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Is a gear rack drive better than a screw drive?

Leadscrew drives are simpler & better for shorter axes. At longer distances they will exhibit a vibration (termed "screw whip") at high speeds.

Rack and pinion drives, which requires a gearing reduction, are better for long axes.

Some CNC machine manufacturers now rotate the nut through a timing belt (after years of claiming how timing belts induce backlash into the drive system) to eliminate the screw whip on the longer Y axis. While an excellent technique for eliminating the screw whip it also means that the bottom of the machine has to be open without any legs supporting the deck. Another disadvantage is that the axes drive is more indirect since the actual drive nut is at least 2 feet from the linear bearing system.

The accuracy of either drive depends on the machine used to produce them. Most gear racks are cut on precision CNC equipment and are actually more accurate than a rolled thread (toothed dies mold the helix into round steel stock) leadscrew.