Unless you are talking about new coins that are yet to be on an exchange, you are better off mining the most profitable coins for your rig. If you want to speculate on other coins that are less profitable to mine, you can get more coins by mining the most profitable coin and trading for the other coins.
Take for example a coin like UBQ, which has a lot less difficulty than ETH and you can mine full units per day with only a couple cards, but on average it's 25% less profitable to mine than ETH. So even though the ETH difficulty is much higher than UBQ, you will be able to accumulate more UBQ by mining ETH and trading for UBQ, than mining UBQ directly. e.g. a single RX 570 mining ETH makes ~16800 satoshis per day and mining UBQ it's only ~12400 satoshis per day. So with UBQ at ~22442 satoshis, it would take you ~32 hours to get 1 UBQ by mining ETH and trading for UBQ vs 43 hours to get 1 UBQ by mining UBQ directly.
http://whattomine.com/coins/151-eth-ethash?utf8=%E2%9C%93&hr=29&p=130&fee=3&cost=0.12&hcost=0.0&commit=Calculatehttp://whattomine.com/coins/173-ubq-ethash?utf8=%E2%9C%93&hr=29&p=130&fee=3&cost=0.12&hcost=0.0&commit=Calculate