I try to be brand agnostic but i'll be the first person so admit that It's hard not to hold biases that you're unaware of.
I've owned Matrox cards, 3dFX cards, Nvidia cards, ATI/AMD cards...at some points in my life I've favoured price/performance ratio and sometimes I've favoured just pure brute force power. Most of the time I've ended up with Nvidia.
Right now I like their 3D stereoscopic support, I like that they're doing G-sync, I like that they have optimisations for wearable devices like my Rift, I like that their drivers have typically given me far less issues over the years than AMDs have, that over the years SLI has always exceeded Xfire despite owning both multiple times. I like their partnering programs with devs and to some extent I like PhysX.
I've owned AMD and I've done AMD Xfire a few times becausae the price/performance ratio has generally been better in the mid range before making Xfire rigs tempting but the older issues with microstuttering made me regret those purchases and pretty regular issues with their drivers and game support always irritated me.
I do think AMD have a lot of Pros though, I'd love to own one again just for the raw speed at mining and hash cracking, they're much faster than Nvidia cards, but right now I got a 980, one of the most impressive pieces of kit on the planet when it comes to graphics.
I'm still open to owning cards from both camps though and make an active attempt to investigate flagship products from both camps whenever new purchase time rolls around.