I would suggest you go the nvidia route, and i think getting a card with more ram is more important than the most cores. Ram is a hard limit that can stop you from doing certain things, but the speed of the card just gets you longer coffee breaks :-D
BUT, if you change to a Nvidia card from a AMD, then make sure to remove ALL drivers and any hints of any AMD in the system, or else you might get some crashes. i learned that when i temporaily had an AMD card in while waiting for the Titan Black to be released, i forgot to remove something AMD related and i got a few hard crashes. Removing all those AMD things and reinstalling the latest Nvidia drivers and i have been rock solid for months now :-D
What exact card that is best value i dont know, somebody else will chime in with good info im sure, but just wanted to say the importance of clearing all AMD if you switch to Nvidia.
Robert