The "tion" ending is much more common and comes from the Latin suffix "-tio" or "-tionem," meaning "act of" or "state of." This ending is used for most words derived from Latin, like:
* Action: "the act of doing"
* Creation: "the act of making"
* Education: "the act of teaching"
The "sion" ending comes from the Latin suffix "-sionem," which also means "act of" or "state of." However, it's less common and mostly found in words derived from French, which in turn borrowed them from Latin:
* Decision: "the act of deciding"
* Invasion: "the act of entering"
* Revision: "the act of reviewing"
So, there isn't a hard-and-fast rule, but here's a good general guideline:
* If the word originates from Latin, it's more likely to end in "tion."
* If the word comes from French, it's more likely to end in "sion."
However, there are exceptions:
* Some words borrowed from French later lost their "s" and now end in "tion," like "tradition."
* Some words that seem like they should be "sion" are actually "tion," like "concession."
Ultimately, the best way to know is to look it up in a dictionary. It's always a good idea to double-check, especially when writing formally!