I think it depends on which coin you want to mine. AMD GPUs such as: RX 480/580 are cheaper but they just achieve good hashrate on Ethas (ETH, ETC, UBQ, ETN...) and Crytonight (XMR) while GTX 1000 series for example GTX 1070 could be very flexible on mining most of altcoin with very high hashing rate, actually the trade-off is its price is a way higher. For me, I choose AMD because I like ETH and XMR.