If I remember correctly, steel-cut oats retain just a little bit more fiber than rolled oats. It's probably not a significant difference, though.
Don't eat pre-flavored instant oatmeal packets, as they'll be full of sugar and god only knows what else. Plus, they're kind of yucky.
Here's the ingredients list for one of the flavored Quaker instant oatmeal packages:
WHOLE GRAIN ROLLED OATS, SUGAR, NATURAL AND ARTIFICIAL FLAVOR, SALT, CALCIUM CARBONATE, GUAR GUM, CARAMEL COLOR, NIACINAMIDE*, REDUCED IRON, VITAMIN A PALMITATE, PYRIDOXINE HYDROCHLORIDE*, RIBOFLAVIN*, THIAMIN MONONITRATE*, FOLIC ACID*.
Here's the ingredients list for Quaker original instant oatmeal:
WHOLE GRAIN ROLLED OATS, CALCIUM CARBONATE, SALT, GUAR GUM, CARAMEL COLOR, REDUCED IRON, VITAMIN A PALMITATE.
And here's the ingredients list for Bob's Red Mill steel cut oats:
Interestingly, that's also the ingredients list for their instant rolled oats. I know which brand I'd rather buy, although I get my steel-cut oats from the bulk bins at Berkeley Bowl.