Stewart wins ....and should win with a good amount of ease. He is a GL after all, even if he is not portrayed as being in the same caliber as Hal or Kyle.
Now, let Steel be wearing the Aegis Entropy armor and he could probably hold off a battalion of Green Lanterns all by himself, and most likely obliterate them.
But in a 'normal' setting, with 'normal' armor, Steel has nothing against a Green Lantern (unless Stewart just stands there acting dumb while the hammer, which increases its kinetic force the longer it flies, comes hurtling towards him. Unless such a level of PIS occurs there is no way Stewart should lose).
However, again, make the armor Aegis Entropy and Stewart will need the Guardians of Oa to come rescue him (and any other non-Ion/Parallax GLs in the sector).
Didn't steel have a power for a while given to him by lex luthor where he actually turned to steel?
__________________ Quotes from Hia8:
"I claimed that the science is sometimes faulty."
"You don't understand. This is fiction. That means none of this stuff really happened."
"There is no writer to purposely ignore a character's natural ability just because it suits the story."
"in some cases because the writer knows that Character A will dominate Character B easily and refuses to allow this to happen for the sake of the story."