I personally find these comparisons with Starship to be way off the mark.
The design philosophies are completely different.
SLS was built from the ground up to be a man-rated system for moon missions. It has no other purposes and needs to work from the first launch.
Starship is an incremental project that is intended to culminate into a man-rated multi-purpose space vehicle but that will take a lot of time and is meant to slowly build towards its ultimate goals through multiple unmanned launches, mostly supporting SpaceX's own Starlink system.
For all the Musk fanboyism, the Starship project is so ambitious that it would be presumptuous to assume its eventual fate based on SpaceX's previous success with F9 alone. The engineering realities of a fully reusable vehicle of this size are extremely sobering, and I think even SpaceX knows this.
It is obvious to me that Starship's underlying urgency with SpaceX lies only in their reliance on it to launch enough hardware into orbit to make Starlink a viable venture. For all their progress since they began assembling stainless steel grain silos in the middle of nowhere and strapped rocket motors on them, they are still a long way away from their goal.
We've been hearing promises of a launch for a while now. They first used the FAA's environmental review as an excuse for the delay, but it is now clear that they were nowhere near ready and, for all the continued promises, who knows how close to it they really are now. The latest rumors is that they are now looking at making the upper stage (Starship) expendable initially, hinting at the fact that they are encountering quite a lot of difficulties with the reentry systems. That and other hints like having issues even keeping the tiles from falling off at every single test firing gives an idea of how far they still are from a rapidly reusable orbital vehicle.
Even if/when they manage to finally make it as usable and re-usable as they aim, it will take a long string of uneventful launches and recoveries before they allow any humans on there. There is also the fact that they need to design a habitable and survivable cabin for it, which is no small endeavor. Despite the Musk-generated hype, we are many years away from seeing anyone climb into one of these things.
Whereas we know how much money SLS is costing, being a publicly funded venture, we have no idea how much money SpaceX is shoving into the Starship program. Incremental design at this level cannot be cheap and it will only happen because they intend to finance it through Starlink. In fact, if you ask me, Starship first and foremost exists for Starlink.
Much as is the case for the Tesla truck/semi/roadster, usable FSD, all his AI/robot nonsense, neuralink, tunnels, solar roofs etc., Elon is very good at making outlandish promises without filing in the obvious technical blanks but, because of his previous successes, people take his word for gospel.
He is a marketing genius well before an engineering one...
I have no conviction whatsoever that NASA would have been better off, in terms of costs or schedule, waiting for SpaceX or even Blue Origin to make a moon rocket for them.
Making a man-rated translunar rocket for the kind of missions NASA aims to accomplish and that needs to be fully operational from launch #1 is going to be a long and costly endeavor any way you go about it. I am sure it could have been more efficient and quick without contractors that have obviously become a bit too comfortable suckling at the government's teat, but not convinced that it wasn't the way to proceed.
At the moment, Starship is still vapourware.
At the moment SLS is still vaporware.
Elon has made numerous predictions which all failed to materialise
The arguments against SLS aren't in any way boosterism for Eldon Musk, and aren't about Space X.
For me, it is a question of using last century's technology vs building the future Space program.
SLS is a dead end.
Neither SLS nor Starship are vaporware. SLS is a small technical issue away from being operational. Starship is at the early prototype stage and a lot of the design and infrastructure has been done.
That said, SLS is much, much further along towards launching humans into space than Starship is.
SLS may be a technical dead end (only if reusability at these kinds of scale and energy ever become doable, which is far from a thing yet), but at the moment, it is the only way to send humans to the moon.
In 20 or 30 years, I'm sure there will be better ways to do it thanks to the increasing commercialization of space and the advent of leaner engineering companies funding new technologies to take advantage of these new markets. Until then, if you want Americans on the moon, SLS is the way to go.