Well a card with more vram would be beneficial for higher resolutions, so if you are looking that these models then if you can get 8gb 570 over the 4gb model or the 6gb 1060 over the 3gb model.
Please post the model number of your power supply, even with the same wattage power supplies can vary dramatically depending on the quality.
It's a good idea to look online and compare second hand cards vs what you can get new as you maybe able to get more bang for your buck, but often people are asking silly prices for second hard parts that are likely also out of warranty, so dont over pay.
You don't need us to tell you what card is better, there are many comparison sites which with give you an idea how cards stack against each other, so you just need to buy the best one for 175 bucks.
Let us know what you find, so myself and others here can try find you a better deal.