Depends on your budget but i can tell you right now this can be a better option than both those cards and it's cheaper get the gtx 480 http://www.newegg.com/Product/Product.aspx?Item=N82E16814130759 which is as good as gtx 570 when overclocked only thing is it runs hotter and needs allot more power so what is your psu it's very important you tell me this before you buy it.
but you can get this one if can afford more $40 it uses way less power then gtx 480 and stay cool and overclock very well once you overclock it it can reach hd 7870 level or more or less.
Depends on your budget but i can tell you right now this can be a better option than both those cards and it's cheaper get the gtx 480 http://www.newegg.com/Product/Product.aspx?Item=N82E16814130759 which is as good as gtx 570 when overclocked only thing is it runs hotter and needs allot more power so what is your psu it's very important you tell me this before you buy it.