http://www.diffen.com/difference/Catsup_vs_Ketchup
According to this thing that's on the internet, "catsup" is the preferred term in parts of the south.
Anyone who says "catsup" should be thrown into a woodchipper with the people who say "pop" instead of soda.In the 1800s, "ketchup" was most common in Britain and "catsup" was most common in the US for reasons unknown. The two words never really canceled each other out because in their formative years, there weren't spelling dictionaries choosing a "correct" version of words. (Many Americans pronounced "catsup" the same as "ketchup" in any case.) Today, "ketchup" is the dominant term in both countries, though "catsup" still has its strongholds, especially in the southern US.