SLS and BFR (now called Starship), are like chalk and cheese.
SLS is a hugely expensive jobs programme. The rockets, when they launch, will be very expensive and single-use. The US taxpayer is paying for it.
SpaceX's Starship will be more capable, cost much less to develop, and will be much cheaper to launch, and should be fully reusable (both stages). SpaceX are spending their own profits (and investors money) on it.