both these chains will have the same problem because this thing called the ethereum difficulty bomb was inserted in the code in early stages to plan for times like this. it is there to increase the difficuly slower at first but a lot as time goes by and bigger when casper was released (the switch to POS) and death of mining practically.
there is a valid concern among miners specially at this time that we are seeing the difficulty rise. so if you already have miners then enjoy the profit but i don't think investing in new ones to mine ethereum is such a good plan.
good news is that change is a hard fork and may cause another split like before and you can end up with two sets of coin, even if the other chain lasts for a short time you may be able to dump it for additional profit.
You will still get ROI if you buy an RX 580 right now and mine ETH (or any of the most popular coins for that matter).
You can sell that RX 580 later and buy Vega and you'll come out with a profit.
But there are a few other coins to mine which are profitable as well, such as ZEC/ZCL, which work quite well on Nvidia cards too. It's not all about ETH.