The only reason USDT seems "stable" is because it claims to be backed by real USD with a 1:1 ratio. But is this claim true? There has been numerous doubts about this recently. Their lack of public audit is fueling these rumors. Some say that free printing of Tether money (that's not backed by USD) is pumping the BTC price to this level. They even confessed that their relationship with an auditor ha been "dissolved" https://www.coindesk.com/tether-confirms-relationship-auditor-dissolved/

So until they have better and more transparent audit I would stay away from them. Just make BTC as your base assets.