I like Nvidia but the problem is Nvidia does not put out very good lower end cards. On the other hand AMD puts a lot of different models and sizes and they are very good and they use a lot less power then the Nvidia.
So My choice would be AMD. I would recommend a card but I don't know what you have. You could have a laptop as far as I know.
Or you could have a slimline case. I just don't know. If you wanted us to recommend a card we need to see your spec's