should I use my iPhone or my iPad to transfer my data to my new iPad.
I'm not sure how to set my new iPad up. Should I use my iPhone or old iPad?
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Whether you choose you iPad or iPhone as the source is largely a matter of preference - but if you are replacing an iPad with another, it makes sense to use the iPad as settings appropriate to the device are transferred along with your data.
These support pages should get you started with setup of your new iPad - and the methods of transferring your data from old to new iPad:
Set up your iPhone, iPad or iPod touch – Apple Support
Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support
Use Quick Start to transfer data to a new iPhone, iPad or iPod touch – Apple Support
If the donor iPad, from which you are transferring your data precedes iOS12.4, QuickStart will not be available.
“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the new device. Nothing is moved or removed from the original device.
An alternative is to restore either an iCloud or iTunes backup to the new device:
How to back up your iPhone, iPad and iPod touch – Apple Support
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support