If your concern is only for trading,you can use both BNB and BTC in binance trading because both are good coins to trade there. I personally used both in trading with different pairs just for short term trading which is turns good so far.Also binance has a full support to upgrade the value of their own coin like bitcoin. But it's still up to you if your going to use BNB to trade but bitcoin i think it should always be your first choice.