R390 or GTX 970?

I've recently assembled a new gaming PC (i7 6700k, 16 GB DDR 4 2133 RAM, Gigabyte Z170 Mobo) and all that's still missing is a graphics card (using an old Radeon HD 6850 for the time being).

My friends told me to get this one:

https://www.amazon.com/gp/aw/d/B00NNXVPS2/ref=mp_s_a_1_4?qid=1464550322&sr=8-4&pi=SL75_QL70&keywords=gtx+970

But this one has twice the VRAM and seems to perform on the same level otherwise:

https://www.amazon.com/gp/aw/d/B00ZPOAYG6/ref=mp_s_a_1_1?qid=1464550424&sr=8-1&pi=SL75_QL70&keywords=r390

They loathe AMD cards for some reason, but I can't tell if it's justified or just fanboying.