I Say See-Quel You Say Es-Quel
While it's generally accepted that either is correct, the different pronounciations do cause a minor problem when using the word with the indefinite article "a" or "an".
You may be saying: "well it starts with an 'S' so it should always be 'a'"; however, if a word that starts with a consonant produces a vowel sound, the correct indefinite article is “an” i.e. “an XML document.”
I guess in the grand scheme of things, this really isn't a huge deal, but it is just one tiny example of how a lack of standarization with something relatively new can cause problems against rules that have existed forever.
Plus, it may distract the reader when viewed in print, forcing him to temporarily change the way he pronounces it or send an e-mail to the editor pointing out the “mistake.”
Or, look on the bright side, either is correct, as long as the usage remains consistent.