Well, in my experience between the two are actually very different.
In looking at the ingredients at my local supermarket I noticed that Ketchup has a lot of sugar and salt in it, where Catsup is made up almost entirely of fruit juices.
This seems to mean that Catsup is much healthier for you than regular Ketchup, and in my opinion, tastier.
Where they came up with the names though, I have no idea.
That only proves that you are looking at two different brands.
The fact, by the way, that original catsup/ketchup had an entirely different recipe is the reason that the formal name of the American sauce is usually given as “Tomato Ketchup/Catsup”.
I was told that Hunts and Heinz used opposing spellings - part of the ketchup/catsup wars. Ketchup won. Did Hunts or heinz win, I don’t remember. 
Heinz is definitely the #1 brand in the US.
If I read Cecil correctly, the original ke-tsiap was not made out of tomatoes (which were unknown to the Chinese – at least before Marco Polo’s visit), but out of fish.
I do not put much stock in the legend that catsup was originally cat soup.
I’m a little uncertain about original ketchup. I was always told it was a Malayan mushroom sauce. But whatever it is, “Tomato Ketchup” was originally a construction parallel to “Soy Hamburgers”.
Tomatoes are a New World food, which were unknown to Marco Polo also. (Even if he actually did make it to China, which is not beyond doubt.)
I don’t doubt it. I was just talking out my ass there anyway.
<closes eyes> Marco!
Yells at Beware of Doug POLO
I’m pretty sure that the Ketchup/Catsup is a generalization of the term. There are actually many different types. Bananas, Mango and other fruit is sometime a main ingredient in these, and I’ve heard they’re quite good. But it’s like out BBQ sauce labeling. There are MANY different types of this sauce and as a general we just put BBQ sauce on it here in the states because we only have one major kind, whereas in different countries they have different kinds and label it as such. Same with the different Ketchups. We just tend to generalize everything into something that isn’t too difficult for the masses to understand.