Both are perfectly fine for payments, I would say. However, it still depends on what you are currently holding. I mean, if a person uses bitcoin as store of value, then he might choose to make payments in the same asset. Otherwise, he will need to exchange it for another cryptocurrency like Ethereum to make a transaction. It requires some time and fees to make it happen. If there is a convinient way to switch assets quickly then I don't see a problem. But again why would he do that if he can make payment with bitcoin. You might say that what if he is holding several assets to make payments. Yes, but that doesn't work when we are talking about storing of value. Ethereum, in this case, must be also considered as store of value, in order for a person not to lose his money while waiting for a good time for a fast transaction.