For me i go with trading ETH/USD since one is a coin and the other is fiat, in which it is balance in a way if eth in general is dropping USD is stable, as for ETH/BTC is risky in the sense if eth is dropping and btc is dropping as well you will be in a very big mess.
In my opinion, advice like that is also more appropriate because it will make it easier for us to calculate every profit and loss that we experience when trading. Because most traders prefer a way that makes it easier for them to trade, choosing the ETH/USD or BTC/USD pair is better than pairing it with an unstable cryptocurrency. Apart from that, there are many cryptocurrencies from various exchanges that are paired with stable coins so that it will make it easier for traders when they want to move their assets from one exchange to another.