The modified for steel will have less constriction. (more open)
Since steel does not compress like lead shot it needs a slightly more open choke. You may run the risk of damaging a barrel if you were to use a full for lead choke while shooting steel. Sometimes you'll see chokes marked "Full for Steel/Modified for lead"
I've had the best luck shooting #2 3" steel out of a Improved Cylinder Remchoke. Which is basically a "modified for steel" choke.
If you know any seasoned trap or sporting clays shooters they can give you more info than you'll ever use about chokes and the amount of constriction thats required to make a given load pattern the best.