I think that it will depend on your strategy.
But for me, I prefer to use pairing ETH/USD than /BTC. Because using BTC pairing will be quite risky for me, moreover if I am willing to have it for the long term. In addition, USD is more stable so we will know the up and down price at least. On other hand, we also need to consider the fee of trading using the pairing. Sometimes, there will be different rates or fees in some exchanges. But sometimes, it is lower or the same.
Look at Ethereum /Bitcoin (ETH/BTC) price chart and you will find an answer.
ETH/BTC history. In 2018 bull run of Ethereum, it got a highest value on market in ETH/BTC when 1 ETH was exchanged to about 0.15 BTC. Now, you see how it is lower than in 2018 considerably.
If you want to invest in Ethereum, invest in it with your fiat currency or stable coin, don't sell your bitcoin or use your bitcoin to buy Ethereum. It is true for all altcoins.