When the dictionary lists two alternate spellings of a word, should you use your judgment (or is that judgement?), or is there some other criterion for selection? Most dictionaries describe what is, rather than prescribe what should be — hence the alternatives — but they usually favor one form over the other. For both convenience and consistency, follow the dictionary’s indirect dictates.
In printed dictionaries, the preferred form will have the full definition, while the runner-up will be cross-referenced to the winner. Online, the spelling in the Web page’s heading indicates the preference, though the other choice will likely also be listed. Here are some common entries with more than one orthography:
1. Acknowledgment/acknowledgement: Acknowledgment, though it looks awkward because the spelling implies that the g is pronounced hard, rather than (correctly) soft, is the preferred spelling, at least in American English.
2. Adviser/advisor: Adviser is the preferred spelling, though it is inconsistent with the spelling of the adjectival form advisory.
3. Aesthetic/esthetic: Aesthetic is the preferred spelling, a rare case of the digraph retained in American English in favor of a single-vowel spelling. (See also amoeba/ameba and archaeology/archeology.)
4. Ameba/amoeba: Amoeba is the preferred spelling. It also has variant plural forms: Amoebas is acceptable in all but the most strictly scientific contexts, where amoebae is preferred.
5. Amok/amuck: Amok is the traditional spelling, preferred to amuck.
6. Among/amongst: The -st extension is, in both American English and British English, widely considered an unnecessary appendage. (The same preference applies for amid/amidst and while/whilst; whilst is, at any rate, rare in American English.)
7. Analog/analogue: Analog is one of fourteen words in which the original -ue ending is clipped. Whether one form or the other is preferred varies depending on not only the word but also, occasionally, on which part of speech it represents. Most one- and two-syllable words ending in -ue have no truncated variant; prologue is the exception.
8. Archaeology/archeology: The version with the ae digraph is preferred over the single-vowel form.
9. Ax/axe: Ax is the preferred spelling, alone and in compounds (axman, battle-ax).
10. Collectable/collectible: Collectible is the preferred variant.
11. Barbecue/Barbeque: Barbeque is a variant of barbecue influenced by the truncation BBQ.
12. Disc/disk: Disc is a variant of disk, though it has valid status in the “phrase compact disc” and references to similar media.
13. Donut/doughnut: Donut is an informal variant of doughnut.
14. Enquire/inquire: Inquire is the preferred American English spelling, but in British English, enquire prevails.
15. Flier/flyer: Spelling depends on meaning. See this post, in which I conclude that pilots and passengers are fliers, and posted papers are flyers.
16. Gray/grey: Gray is the preferred spelling in American English; British English favors grey.
17. Nite/night: Nite is an informal variant of night.
18. Theater/theatre: The former spelling is preferred in American English, though the latter form sometimes appears in proper names.
19. Toward/towards: In American English, towards and other similar words are considered informal variants of the forms in which the s is omitted.
20. Whiskey/whisky: The former spelling is more common in the United States (as well as in Ireland), though usage in labeling varies.