Is it better to do Margin trading in fiat or btc? You can do both right?
If you hold 1 BTC, you can sell that one and another one, so you gain the difference of 1 BTC movement on the way down.
If you hold 1 BTC worth of USD, you can sell 3 BTC, so you gain 3x BTC movement on the way down.
Do not ask me why it is done this way on bitfinex. But you see how holding BTC in bear markets is a bad idea.