Choosing between "...ize" and "...ise"

Explained at DCBlog:

The -ize spelling was preferred by classical scholars, especially in the 16th century, for verbs which came into English from Greek and Latin, and that etymological argument has fostered the use of z ever since. The USA and Canada adopted it from the outset. And the editors of the Oxford English Dictionary opted for it, at the end of the 19th century, partly on etymological grounds (a z is used in Greek and Latin) and partly on phonological grounds (that the letter better reflects the sound)...

So where did the -ise alternatives come from? Some of the words such as baptize) were spelled with both an s and a z from their earliest days in Middle English. The trend to spell all such verbs with s began when verbs came into English with increasing frequency from French, where the suffix was -iser. A verb of this kind borrowed directly from French, it was argued, should be spelled with -ise, to reflect that source. Some felt it important to maintain a spelling link between related words, such as analyse and analyst. And during the 19th century, this usage grew.

The problem, of course, is that it is often unclear whether a verb has come into English from French or from Latin. Confusion led 19th-century printers to try to sort it out, and they did this by imposing a uniform rule for all such verbs where alternatives exist...

World usage varies. -ize is the overall preference in North America; -ise in Australia. Usage in the UK is mixed, with -ise beating -ize in a ratio of 3:2... Some publishers these days are adopting a more relaxed attitude: they don't mind which authors use, as long as they are consistent.

Via Sentence First.