GTX 1660 Ti vs. RTX 2060 for a deep learning pc
I would have gone with the RTX.
If I was to design deep learning models,I would be prefer more number of cores in the GPU , where RTX is the winner.

Lets not forget the Tensor cores which are present in RTX , which makes it stand out for Deep Learning processing. (Faster Tensor operations).

Go with RTX if you can afford it.