they're two different ways of describing the same product.
catsup
Catchup \Catch"up\, Catsup \Cat"sup\, n. [Probably of East Indian origin, because it was originally a kind of East Indian pickles.] A table sauce made from mushrooms, tomatoes, walnuts, etc. [Written also ketchup.]
Source: Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.
catsup
\Cat"sup\, n. Same as Catchup, and Ketchup.
Source: Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.
catsup
n : thick spicy sauce made from tomatoes [syn: ketchup, cetchup, tomato ketchup]