The BFR is a glint in spaceX's eye - not something that meaningful comparisons can be made to a rocket that already exists.
And if the BFR timescale is within 3 years of what Musk has said, I will eat my own hair. I would be overjoyed, just pretty certain that it's not happening.
SpaceX are already constructing the first BFR prototype, so I think the glint in the eye comment might be a little out-of-date. Sure it's not here yet, but neither is SLS. Musk has talked of it being tested next year.
The first SLS to fly will (by current plans) be next year. But it will be the smallest size version, which only just beats Falcon Heavy. It will also be launched as a proof of concept, and not carry anything meaningful (there was some talk of putting crew on board the first launch, but that idea has sensibly been abandoned).
It will take them *at least* another year to build a second SLS, by which time the BFR prototype will have been doing test hops for months.
There are only enough engines stockpiled for them to build four SLS rockets. After that, they need to restart the production line if they want to launch any more. Aerojet have quoted them a billion to do that (not counting the cost of the actual engines that get built) and a build rate that will only allow one SLS launch every 18 months.
If they really want to build this Lunar Gateway station they're talking about, it will take multiple flights, even allowing for it to be built in large "monolithic" pieces, so given the maximum possible flight rate imposed by engine build constraints, it will take them a decade to build it.
Given all the above, BFR can be delayed for *years*, and it will *still* be a better platform and available soon than SLS for most of the missions planned for SLS.