whatever AS's penchant for speechifying, he's much better at distributing moments among the ensemble

Uncontestedly, and also, better at getting excitement about politics across. The Upton Sinclair campaign story per se sounds definitely interesting and worthy of movie treatment (and pointing out the obvious precedents for current goings-on). But the way Mank the movie treats it, in the end, is to give Mank the character a non-petty motivation to go after Hearst as a writer. Which I don't think Sorkin would have done. He'd either made the story much more central or given Mank some intentional ambiguity as to whether he cares about this because he thinks Sinclair is right or whether it's for petty reasons. (Since googling tells me there's zero evidence Herman M. cared about Sinclair one way or the other - though brother Joe did - , it's invented-to-make-Mank-noble subplot anyway, so both fleshing it out or cutting it altogether would have been legitimate choices.

I'm looking forward to learning more through you about the campaign, though, as it was new to me as well.