For the price right now, probably the RX 470. $150 won't get you much from Nvidia. Unless of course you wait, A LONG TIME. I bet 6 months from now 970s will still be well over 250 dollars. 960s..well you those are even worth using. There is of course GPUSHACK.COM, they sell like new gpu's for less than what everyone else sells them for. They are very trustworthy and they come with a 2 year warranty. The cards are spotless. I bought one last year from there and I couldn't tell any difference between a new or used card.
If you are really on a budget, they have ASUS GTX 660's for $99, ASUS 670 DC2's for $119 and MSI 970 OC's for $259..there is another page on their site called "commercial deployment" they have a ASUS 7970 for $179 (all cards come with free shipping if you live in the USA)
You could also buy one from ebay, there are quite a few R9 280x on there for about $140 dollars and they are still decent cards.