I know you're desperate for an answer so please be patient, I've got a life too.
Let's start with the GPU comparison
The GTX 650 ti boost is better than the HD 7790 , but around the price range of the GTX 650 ti boost, you could get the HD 7870 which is better than the GTX 650 ti boost.
So, AMD vs Nvidia ? They're both good.
Comparison
AMD
- generally cheaper
- better in OpenCL
- commonly used for Bitcoin mining
Nvidia
- generally consumes less power ( about 10% less )
- has the best high end GPUs
- PhysX - better fluid physics and stuff , but only for certain games.