While Earth would be easier to terraform due to available resources and global conditions already closer to something inhabitable, it would not be safer, as mistakes in the terraforming process are not going to be as catastrophic when you try to terraform a backup, uninhabited planet.

Toying with complex, poorly understood processes at a time when we wouldn't even have our current resources, manpower on a ravaged Earth whose environment might just be one wrong step from becoming much worse, could destroy a majority of what remains of humanity, the economy and valuable resources, making it impossible for us to ever recover.

(I am however assuming we were talking about global terraforming of the whole planet, not making minute changes to local spots)