Hi-
I believe the difference is how the whole oat grain is cut... Rolled Oats are "flaked" and steel cut is kind of pulverized into little "grape nut" like looking particles. Rolled oats are also slightly steamed during manufacture so that they cook faster than steel cut.
Nutritionally, old fashioned rolled oats are the same as steel cut... its more about the texture, than anything.
Personally, I prefer rolled oats.. they cook faster and seem like you get more food in your bowl post cook. But, that's just my personal preference.
Lynn M.