The Tischendorf story is entirely plausible and its subsequent retellings have few serious variations, even if Tischendorf glosses over stealing the Codex from the monestary.
The Simonides story, on the other hand, is very convoluted and, as time progresses, gets more convoluted and improbable and even contradictory.
Logic dictates that the Tischendorf story. or at least one version of it, is the truth.