I've not tested a huge amount on casting charms, as i focused on weapons mostly, still i've recorded more results than anyone else that i'm aware of seeing no-ones really done any testing on charms with the possible exception of Meridius whom no longer plays.
It's known you start with roughly a 20-25% chance to crit with charm casting. This only increases as your intel increases.
On weapons, +crit items consistently give very close to +1% chance to crit per +1 crit. From some early tests this does still seem to hold true, however i haven't got enough data to be 100% on this at this stage.
Would expect a mage at say level 10 with 0 crit gear and reasonably good int gear to have ~25% chance to crit. It's certainly plausable that you could argue +1 crit is equal to +2 int at lower levels.
+1 crit however significantly worse than 3 int at low level
As damage is pretty exponential so does crits potential, so at around level 55, it gets close to +3 int per crit.
I would argue strongly that the 50 int dagger will significantly outdamage the 32 int 5 crit staff. Especially at lower levels. By level 70ish, it will be somewhat close.
If you have recorded any testing to prove otherwise i'd be interested to see it.