Once it hits your digestive system and the respective enzymes (starting in your mouth), they all start to break down to monosaccharides pretty quickly. The problem with these refined carbs is they (A) Don't come along with a lot of the nutrients you would find in leafy greens, natural cereals, and (B) They are frequently missing a lot of fiber which means they all basically get digested and hit your bloodstream.
I'd love to see a comparative study on the relative difference (or lack thereof) between (A) Refined Cereal with Sugar. (B) Refined Cereal zero Sugar (C) White toast.
My sense is your body basically treats them all the same at the end of the day, but I'd still love to see the study.
The studies you're looking for are related to glycemic index, I think..
Glycemic Index Value: (from http://www.gilisting.com/glycemic-index)
Corn Flakes: 82
Froot Loops™: 69
White Flour Bread: 71Glycemic Index is really complicated because it can be affected by what else you eat with the food (like putting milk on cereal), or how you prepare the food (some people think toasting bread can lower the GI).
You're right that "how much sugar per serving" is an oversimplification - I just use that as a starting point because most nutrition labels don't list GI. And even if they did, the number isn't really meaningful except in the context of everything else in your meal, and whatever else is going on in your body that day.
Anyway, next time you're at the store, try to buy cereals with <10g of sugar and see how limiting that is.