If it says +11.79% is it better to sell or buy usdt then?
Is it then better to hold onto the BTC I have for it to increase before I trade it against another currency?
This means is BTC is down 11.79% from 24 hours ago. If you hold BTC 24 hours ago, this is the value now. If you dont have any btc, then it means it is cheaper to buy btc now compared to 24hours ago
It is up to you either you hold BTC to sell at better price , or you want to sell it now to prevent further losses