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).