Which is less credible: controlled demolition of WTC, or flat earth?
There are two distinct aspects to this: the physics, and the politics.
On the physics side, controlled demolition is much more credible (or less incredible, if you prefer) than flat earth. Unlike flat earth, controlled demolition of a building is physically feasible. And at least superficially, the collapse of the Twin Towers, and WTC7, does look something like controlled demolition. The difference between the two hypotheses is mainly in the initiating mechanism, not the subsequent process of collapse. The main physical objection to CD is that demolition by means of explosives makes very loud bangs, which were not heard or recorded at the WTC, and that subsequent investigation did not find any of the tell-tale signs of CD, such as explosive residue or fuze materials. (This may be why some 'truthers' have gone over to more exotic hypotheses like nanothermite. Of course, truthers may also reject the investigation findings as a cover-up.)
On the politics side, both controlled demolition and flat earth involve conspiracies, and are open to the usual objections to large-scale conspiracy theories. In the case of controlled demolition, conspiracy is essential to the hypothesis. The heart of the hypothesis is that malign actors (the US government, the FBI and/or CIA, the Rothschilds, Mossad, the Saudis - take your pick) conspired to plan, finance and execute a controlled demolition that was bound to kill many people. The main objection to the conspiracy theory is that it would involve a large number of people to carry out a very complex operation without being detected, or disclosing the truth, either before or after the event. There is also the major problem of how to integrate a theory of controlled demolition with the suicidal hijacking of planes by unreliable people. (Unless you are going to go full-on crazy and argue that the planes were holograms, or remote-controlled drones, or whatever, and the 'hijackers' were just actors. )
In the case of flat earth, conspiracy is not essential to the hypothesis, but in modern times it might be called an unavoidable by-product. One cannot now be a serious flat-earther without believing that all the world's governments, space agencies, airlines, scientists, and many others, are part of a conspiracy to cover up the truth that the earth is flat. In terms of the numbers involved, the flat earth conspiracy would have to be far larger than the WTC demolition conspiracy. The latter might conceivably be managed by a small team of expert operatives, perhaps no more than a hundred, whereas covering up flat earth would take hundreds of thousands.
So I think that overall flat earth is much less credible than controlled demolition, on both physical and 'political' grounds. But as Dr Johnson once remarked: "Sir, there is no settling the point of precedency between a louse and a flea."