Steel cut oats or stone ground oats are slightly healthier then instant oatmeal because they have a lower glycemic index. With steel cut oats, that oatmeal is just broken in half where as with instant oatmeal it is rolled and steamed. This makes the oatmeal cook faster but also digest and raise blood sugar levels faster.
It's kind of nit picking really but stone ground oats are slightly superior to instant oatmeal because they will raise blood sugar levels slower.
I personally like the taste of stone ground oats better. They have a more nutty taste to them then instant oatmeal.