I use both for hotdogs. However, I only put mustard on hotdogs while I put ketchup on many things. Such as burgers, fries, hotdogs, and some other foods.
I love Honey Mustard. I use Spicy Mustard every once in a while. I don't use Ketchup very often, although I add some spices to it to make a sauce. I have it with meatloaf.
*gasps in horror* Blasphemy! I really like both and don't think I can't choose between the two. Mustard is good on some things and ketchup is good on some things. I use them both equally. So, both!
Ketchup because... I like the taste better. Then again, I don't really like mustard in general, so I just don't use it. Plus, Ketchup goes better with most things than mustard does.
Awful Joke: A family of three tomatoes were walking downtown one day when the little baby tomato started lagging behind.
The big father tomato walks back to the baby tomato, stomps on her, squashing her into a red paste, and says,
I never eat any of them alone, I have to put them on something. I prefer ketchup because I seem to use it more, like you can put ketchup on lots of stuff which I think some people mentioned like fries, you can't put mustard on fries or it'll be disgusting.