Answer: there really isn’t any!
They are all a nutritious whole grain that offers soluble fiber (the kind that has been linked to a reduction in total and LDL cholesterol levels).
(Quick review: insoluble fiber — found entirely in whole wheat products and partially in fruits, vegetables, and legumes — speeds up the transit of foods in the digestive system.)
The difference between these varieties of oats and instant oatmeal ultimately comes down to processing techniques.
Whereas steel-cut oats are — ready for a shocker? — cut by rotating steel blades into tiny groats, quick-cooking oats and instant oatmeal go some extra processing that produces a flat flake.
If you look at their respective nutrition labels, steel-cut oats appear to contain more fiber than their quick cooking counterparts. However, this is simply due to different serving sizes. It’s akin to a one-ounce slice of whole wheat bread containing 3 grams of fiber and a 1.5 ounce slice providing 4.5 grams. The larger slice may appear to be a “better source” of fiber, but ounce by ounce the two are equal.
While steel cut oats have a lower glycemic index than flattened oat flakes, I don’t consider the difference significant. Additionally, it is more important to consider glycemic loads (how what you add to your oatmeal affects its glycemic index).
You can’t go wrong by buying plain (unsweetened, no salt added) oatmeal and jazzing it up with nuts, seeds, spices, and some fruit. Additionally, cooking it in milk (dairy or otherwise) adds protein and additional nutrients.
The problems begin when you buy flavored varieties than add sodium and up to 4 or 5 teaspoons of sugar. So, keep it simple, real, and whole!