Its like asking which is better, Tea or Coffee? They perform similar functions and are both hot drinks. Price and quality varies between manufacturer also, so each product needs assessed on an individual level.
You cannot definitively say one is better. You can compare individual cards but it still comes down to what your personal preference is and how much you're willing to spend. I prefer Nvidia, because I like low temps and the 600 and 900 series are great for that, my friend however has a AMD tri-x 290 that is blazingly quick but runs what I would consider hot.
The choice is yours!