The good news is the volatile nature if Crypto Currency does occasionally leave a small window when High End GPUs become available again... GTX 1070's & 1080's showed up in stores and miners are hesitating to scoop them because of the down turn.... but Bitcoin will go up again soon.
It depends on the card. my bro got his 580 last year for like $250 and try to get one at that price now. He's kind of lucky I guess if you're a fan of 580s. I personally think Nvidia has better stuff and is usually easier to find lately.
Your thoughts are right... nVidia Cards are definitely better... have been for a many years now. The thing is they're prices are really Jacked up so unless ypu have a bottomless budget one can't help but peak at AMD Cards.