Here's why I think SLS Block 1B is a better vehicle than Starship for Mars missions: First of all, I think we should utilize the Moon's resources for Mars. Starship can't be used for that, since it uses CH4 as propellant and there is almost no carbon on the moon.

SLS uses liquid hydrogen and that infrastructure is present at LC-39B. A hydrogen tanker vehicle can be sent between the Moon's surface and back to orbit, to fuel Mars Transfer Vehicles. Something like the XEUS concept from ULA. Launching this on an SLS means that we could...

...launch it directly to the moon without refueling it in obit before. Using liquid hydrogen and constructing it around the Moon (perhaps at the gateway) means we would need a much smaller propellant tank overall, which means that we could have bigger habitats.

And in the more distant future I think we should move over to a nuclear engine architecture. This would allow for even better efficiency.

And then we have the crew launch: SLS will launch Orion, which is a much smaller and more efficient vehicle. SLS/Orion also has a launch abort system compared to Starship which doesn't have one. Eventually Starship may become safe, but abort systems will be needed...

...for many years to come.

For the Mars landing part, we should send a lander directly to Mars on an SLS Block 1B. Lockheed Martin's MADV is a great concept. This vehicle will refuel LH2/LOX on Mars:

Starship has to do a "suicide dive" to Mars' surface, which MADV doesn't need. This is a much riskier approach than entering orbit before descending.