Which graphic card for developing mobile games on iMac? Ati or Nvidia?

Hi!

Now, there is a time to replace my old macbook because this old hardver is not fitting my ... and Unity 3D needs for development. I wan't to buy a 27" iMac becasue I'm using a secondary display and a graphic tablet too. The iMac with retina displays only have Ati graphic card, but the old one (with no retina display) still have Nvidia chipset.

The problem is, that now I can buy from a local store an iMac with 27" retina display and with AMD Radeon R9 M295X 4GB graphic card + 16GB of RAM nearly for a same price as an prev.gen. 27" iMac with NVIDIA GeForce GT 755M 1GB + 8GB of RAM from the Apple store.

Which one is better for iOS and Android game development?

Thanks!