Legacy Intel iMacs

iMac 2013 27" VS iMac 2017 21"

https://technical.city/en/video/GeForce-GTX-780M-vs-Radeon-Pro-560

https://technical.city/en/video/Radeon-Pro-560-vs-GeForce-GTX-780M-Mac-Edition

• GeForce GTX 780M (4GB) is the best available option for a iMac 2013 27" (the 27" model only has a late 2013 version)
  (supports DirectX 12, but the feature level is only 11_0)
  Also see https://www.techpowerup.com/gpu-specs/geforce-gtx-780m-mac-edition.c2526
• Radeon Pro 560 4GB is the best available option for a iMac 2017 21" 4K