The plastic one is stronger than the aluminum one. Aluminum cracks under a Connies vibrations while the plastic one will not.
That "plastic" rack has held 100lbs steel components in a GIVI case running around Brooklyn cobblestone streets just fine. I have never had one fail.
Go ahead and use it, no need to go out of your way for an aluminum plate.
Also, the aluminum plate does not bolt onto the base mount of the GIVI rack setup.
PS I had a few broken GIVI aluminum plates in the past.