Picking a graphics card depends on what you are planning on doing with your computer.
- Play games
- Mine bitcoin
- Play super intensive fast pace games
- Surf the internet
- Watching youtube videos
Once you have answered which one or more of these you will need a graphics card most of the decision is done. The next question is do you or your games need cuda support. Cuda is a parallel programming library developed by Nvidia and runs exclusively on Nvidia. Thus if you need Cuda your decision of which GPU vendor has been decided.
An up and coming alternative to Cuda is Vulkan. Vulkan is a cross platform computing API which is intended to replace OpenGL or Direct3D and also offers parallel processing.
If Vulkan actually becomes more common it could allow more and higher quality games to be available on other operating systems than windows.
The final piece or the decision is how much money can you afford. Both Nvidia and AMD do have graphic cards that cover the price spectrum. Usually this is selecting either a slightly older model or the latest and greatest GPU. It is just a matter of getting the most GPU that you can afford that fulfills what you need a computer for.