Versus #6!

[eggs]

Spellthief probably does more damage* (2d6 sneak attack taxes IP pretty hard trying to match, even without Craven), but in almost every other way, Factotum has a pretty serious edge (more spells, higher-level spells, wider selection of spells, respectable save/skill/opposed Str bonuses, better skill list, the turning/healing thing).

*There's IF, but that involves several layers of investment and ignoring the fiction the skill models, plus it fouls up a character's action economy to try to force in rounds after the first.

Spellthieves' major abilities make cute tricks (especially with specifically spellthief-accommodating party members) - but they aren't especially reliable (maybe one in twenty CR 5-9 monsters has an SLA to steal that's worth stealing, spell-lockdown is only at Sorcerer rates and relies on sneak attacking some of the hardest classes to sneak attack).

But as usual, looking at a 1v1 arena scenario, it's mostly about whichever hits the other early and hard. Factotum's Int-to-AC/Initiative make it less likely to be the one to go down first.