-tion or -sion
Both spell the 'shun' at the end of a word, and -tion is far the commoner. The word the noun comes from usually says which.
The rule
-tion and -sion can both spell the 'shun' ending. -tion is much the commoner; -sion usually comes from a related word ending in D, DE, SE or SS (decide, decision), and a 'zhun' sound is almost always -sion.
-sion usually follows a related word ending in D, DE, SE or SS: extend, extension; decide, decision; confuse, confusion; discuss, discussion. Most others take -tion: relate, relation; inform, information; complete, completion.
The sound helps too. When the ending says 'zhun', as in vision, decision and explosion, it is very nearly certain to be -sion.
Admit, permit, omit and commit take -mission: admission, permission, omission, commission.
Where it stops holding
There is a third and fourth spelling of the same sound. -cian is used mostly for a person (musician, magician, politician), and a handful of words end -cion, like suspicion and coercion.
Background
Picking an ending from the word it was built on is spelling by structure, which the research finds teachable. That research is on how people learn to spell.
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