wacko already said everything. In my opinion i would go with the 580 8gb they will be usefull in 4-5 years you will easily sell them for technical schools or office pcs which need a little bit of graphic power. I dont know how high your investment is but it also depends on that. Our company is going to invest 50.000 Euro into Miners => GPUs so we are already thinking about days now which is the perfect mining rig for long time use.