That depends on what will be your aim in your trade
If you are wanna getting a stable profit and then you must use USD as your trading pair to the coins and when you are getting bitcoin as your return and you must make your fund available in bitcoin form.
I personally will try to choose the USD over BTC caused by when you can get the stable income from your trade and that will be very good to see that it will be stable and not getting affected by the volatility.
It's better to choose USD. When you are choosing bitcoin in a bearish situation and that would be a very bad thing.