Just wanted to point out why it's hard to learn anything definite from forums like this:
They are NOT "two different accounts" of anything and specifically not of what "THE [implied singular] tragic flaw of the buildings were" )Notice that these are two different accounts of what the tragic flaw of the buildings were: the open floor space vs. the welds. And neither of them has been acknowledged as the explanation for the total progressive collapse by anyone with the expertise and authority to make such judgments.
Jeffrey Orling's comment identifies - correctly - two of the features that dominated the overall collapse mechanism.
Mick West's comment identifies - also correctly - ONE of the key details of the collapse mechanism.
Both comments state partial truths which are complementary parts of the overall explanation The are NOT alternate total hypotheses. And neither member claimed they were total solutions.