Keep with T or change to C in verb to derived word conversion

Solution 1:

-tial/-cial

I found a site that offers a rule of thumb for spelling words ending in -tial/-cial which doesn't even require you to know the spelling of the related verb. It is

Use -cial after a vowel, like after the vowel ‘o’ in social, ‘e’ in special, ‘i’ in beneficial

Use -tial after a consonant, especially, after ‘n’ in substantial, essential, ‘r’ in partial

[...]

Memorise/memorize these seven exceptions: financial, commercial, provincial, initial, spatial, palatial, controversial

(from www.howtospell.co.uk, by Joanne Rudling)

This list of exceptions is probably sufficient for common words. You should know that it is not comprehensive; it excludes for example adventitial, solstitial, preputial; internuncial, uncial; and probably also some other rare words. Despite the presence of these exceptions, this does seem to be a real trend, based on a comparison of words that end in -tial and words that end in -cial (Morewords.com).

-tion/-cion

There are only two common words in English that end in -cion: suspicion (related to the verb suspect) and coercion (related to the verb coerce). There are many words spelled with -tion, which can come after vowel letters (as in creation) or consonant letters (as in perception).

You might also be interested in the answer to this question about when to use the spellings -sion, -⁠tion and -cion.

Solution 2:

The first poster was right (but not helpful) when he said this is an 'irregular' formation. So I don't have a rule to give you, but at least there is an explanation for the irregularity; some forms of the words benefit have always had a 'c' in them.

http://www.etymonline.com/index.php?term=benefit&allowed_in_frame=0

So this isn't a weird case of adding a 't', it's a case of benefice losing its 'c' over time.

http://www.etymonline.com/index.php?term=beneficial&allowed_in_frame=0