This is a tough one, because they're both highly problematical episodes from a conceptual standpoint, yet both have points in their favor.
The Phylosians in "Infinite Vulcan" are fairly interesting, but the rest is flawed. Even aside from the giant-clone nonsense, Keniclius's backstory is hard to reconcile. He's a product of the Eugenics Wars in the 1990s, and yet his plans called for spreading supermen across the galaxy? How did he even get out into space? And the chronology is screwy -- at one moment, Keniclius reveals knowledge of "the galactic wars" and the Klingons and Romulans, but in the next, Kirk is saying he's been out of touch for 200 years. True, the discrepancies are a product of when it was written, a time when Trek chronology was still rather vaguely defined, but it doesn't fit well with what we know now, and it was only marginally less problematical even when it was made.
"Serpent's Tooth" is ambitious and visually impressive, but as stated above, it is pretty much a beat-for-beat revamp of "Who Mourns for Adonais," and it's got a lot of problems. Considering it was co-written by a Native American screenwriter trying to increase the representation of his people, it does a pretty cruddy job of depicting Native American history and mythology. The legends of Kukulkan described in the episode are nothing like the real myths, and the chronology and history of ancient cultures, both American and elsewhere, are grossly misrepresented in service of the conceit that all Earth cultures were imperfect replicas of Kukulkan's ideal city. And really, how could a pyramid built by ancient technology and powered by mere sunlight have the ability to send a faster-than-light transmission to Kukulkan?
It's hard to give a totally fair assessment since I've watched "The Infinite Vulcan" recently but haven't yet gotten around to "Serpent's Tooth." But I think I have to give the edge to TIV. As strange and silly as some of its aspects are, at least it doesn't do a disservice to history and cultural lore, and at least it isn't a rehash of a story that had already been done.