Buying to mine is good but you might want to wait to see what the 490 does. I myself PoS on a pi which is super efficient but I can't resell my 35 dollar pi. You could buy a new 490 next month when they come out and mine for a month or two and resell it for 10% off your buy price. So its a tough call I suppose... nevermind me hahaha. But if you wait until the 490 drops you will get a better price on the 390x anyways.