iMAC 27 Difference Graphic Processor

Just for my curiosity and knowledge, from a Software and Hardware stand point prospective,

in reference to the iMAC 27 graphic processor brand, what are the difference between:

the NVIDIA GeForce GT 755M with 1GB and the AMD Radeon R9 M290 with 2GB, in terms of speed, performance and responsiveness? And why the iMAC 27 Retina model has the AMD Graph processor and not the NVIDIA processor?

Thank you so much and have a wonderful day.