Ketchup or catsup? My first instinct when writing the word is to use catsup, and I can't really tell you why. Both words mean the same thing, a spicy/sweet sauce made from ripe tomatoes, so what's the difference? Well, when you get right down to it, there really aren't any.
Both ketchup and catsup trace their origin to the mangling of a Chinese term for fish sauce: ke-tsiap. As the condiment migrated through southeast Asia it's name was localized and when 17th century English sailors took to the sauce, the name was further anglicized to compensate for sounds that simply didn't work for the Anglo palate. In the 1800's you could point to the fact that British English speakers were more likely to use the "ketchup" spelling while American English speakers most commonly used "catsup".
Those of you who are fans of Mad Men may have seen the episode featuring a fake sales pitch to Heinz promoting changing the spelling to ketchup in order to differentiate the product in the eyes of US consumers. While that meeting was fictional, according to a Slate interview:
"Henry John Heinz first brought his product to market as “Heinz Tomato Catsup,” but changed the spelling early on to distinguish it from competitors. Del Monte did not switch spellings until 1988, after it became clear that ketchup was the spelling of choice for American consumers. Hunt’s switched the name of their product from catsup to ketchup significantly earlier."
So, at least according to the sources I can find, if you say "ketchup" is right you've got corporate America behind you. If you choose "catsup", well you're an old school rebel and here's to you!