Pound for pound, Flaked oats and Steel Cut oats should have the same impact on body, flavor, and mouthfeel.
Both processes begin with raw, dehulled oats. The "groats" are toasted to halt lipolytic enzyme activity that would make the oats go rancid. Here, the process diverges. Steel cut oats are cut along the length of the groat, giving that small, cylindrical appearance.
For flaked oats, the groats are steamed, then crushed between two rollers to flatten the kernels. After drying, the result is the flaked oats you can buy at the homebrew store, or in the grocery store as "Old Fashioned Oats."
It's important to note that steaming the oats changes the nature of the starch (gelatinizes it), making it accessable to amylase enzymes that convert the starch to sugar. If you're going to use steel cut oats, you'll need to cook them first.
As for the "nuttiness" you mentioned, you can increase it by baking the oats (flaked or steel cut) on a cookie sheet at 300F until, to quote Randy Mosher, "it smells like cookies."
For more info, check out this and this.