Well, first we have to look at the nutritional information on the fries: Which fries are less salty? More fatty? More greasy? Well looking at McDonalds fries (medium size), we can see they have: 380 calories, 29% total fat, 13% saturated fat, 11% sodium, 16% carbohydrates. Burger King's fries (medium) have: 400 calories, 28% total fat (Burger King's site didn't have nutritional information, so not all categories could be complete).
Looking at the current results, McDonald's french fries are healthier, but Burger King's appear to be more tasty. Knowing that both the McDonalds and Burger King establishments use Heinz brand ketchup, we can determain that 9 times out of 10 the average fry consumer would have ketchup with his/her fries.
As you can see under the "Heinz ketchup nutritional information" source, you can see the ingredients and corresponding nutrition that is in them. When eating a meal, typically you will have those little white cups. Assuming your cup is filled to the top, you can (if you really want) as the manager for the size of the cup in oz, then take a calculator and carry the oz of the serving size stated in the source and divide it. You will then get your total nutrition as divded by whichever number it is.