Foreword
Recently a great guide was created that delves deep into math and theory to help you understand whether you should use gear with Flat or % substats. Sadly, the answers isn't simple and the best choice depends on the character itself.
Guide
Since the guide is pretty massive, I won't import it to the website and instead leave it in original form - as a google sheet. The sheet was created by Kitoen#8140 and all credits go to him.
Flat vs % Artery Gear guide