From an average trader's perspective, there's not much difference except that you'll find a whole lot more trading pairs and liquidity with USDT rather than BUSD.
But there is a few differences:
BUSD is regulated and audited. They ensure that every BUSD is backed 1:1 by a physical dollar stored in FDIC-insured US banks.
USDT is not audited and has had security breaches and lawsuits in the past. They don't guarantee a physical dollar backs every USDT and the money that does back it is stored in offshore banks.
So I would consider BUSD to be a safer stablecoin, but USDT is the more useful one for traders due to its huge daily volume and number of trading pairs.