The switching rate in a processor is faster than the firing rate of neurons.
All else being equal, a computer should be faster than an aggregate of neurons. But all isn’t equal, even when comparing different processors. Comparing transistors in a modern processor to synapses in a human brain yields many more synapses than transistors. Furthermore, the brain is massively parallel, and has a specialized architecture. For what it does, it’s well optimized, at least compared to how optimized our software and hardware are for similar tasks at this point.
For instance, laptop processors are general purpose processors, being able to do many different tasks they aren’t really fast or good at any. Some specific tasks may make use of custom made processors, which, even if their clock rate is slower, or if they have less transistors, will still vastly outperform a general purpose processor if they are to compete for the task they were custom-built for.
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)