In all honesty... I have no idea which is better because both lists similar specs. I assume both have hard drives which are 5400RPM. Maybe the G570 uses a lower quality LCD screen, but the only way to know is to view them side by side.
G570 use more low end plastic in the case and is the 'budget line' in features.
Z570 is a mid-range home entertainment model with upgraded speakers and a better case.
Be sure to check other features like what version of Bluetooth, etc, each has.