It depends :-) over the years I have switched between the 2. In the early days I was more pro amd (ATI) and the most recent years I am edging more towards Nvidia.

However it depends greatly on:

• your budget
• the use ( gaming, video editing, Office tasks)

I always start out watching benchmark sites or review sites. A good place to start would be: http://www.tomshardware.com/reviews/best-gpus,4380.html