Pronunciation is different for -al and -ial words
I don't know whether you meant for this to go without saying, but -al and -ial differ not only in spelling but also in pronunciation:
The letter I in -ial often corresponds to an unstressed high vowel /i/ (or in some British accents, /ɪ/) or a glide /j/. Or it may represent a modification of a preceding consonant sound (facial is typically pronounced with /ʃ/, unlike the noun face which is pronounced with /s/).
The stress in a word ending in -ial is always (or at least, almost always) on the immediately preceding syllable, whereas the stress of words ending in a consonant letter followed by -al is harder to predict based on the spelling (there are a few rules that cover particular subtypes of -al words; e.g. if there is a "heavy" consonant cluster before -al, as there is in perichon.dral, the stress tends to fall on the immediately preceding syllable, whereas for words ending in -ical, the stress falls on the third-to-last syllable as a rule).
If the related noun contains I/Y, it's likely that the adjective will also
For some words ending in -ial, the letter I corresponds to an I or Y in the related noun. For example, cranial and cranium, radial and radius, mercurial and mercury, arterial and artery, adversarial and adversary. (There is however an exceptional word peripheral that uses -al instead of -ial despite being derived from the noun periphery which ends in Y.)
If the related noun ends in -ce, it's likely that the adjective will end in -ial
In Latinate English words, the ending -ce often can be traced back to earlier -ci- or -ti-, so many nouns ending in -ce have corresponding adjectives ending in -cial or -tial. Some examples: face, facial; office, official; space, spatial; substance, substantial.
If the related noun doesn't contain I/Y or end in -ce, I don't know of a good rule for determining whether to use -ial or -al
For some -ial words, such as your example of editorial, there isn't an I in the related noun (editor). It is hard to say why we say editorial but doctoral. In fact, doctorial is not entirely nonexistent, although it's very uncommon. As far as I know, the fact that we usually say (and write) doctoral instead of doctorial is somewhat arbitrary.
Neither -al nor -ial is very productive as a way of forming new adjectives in modern English.