The steel frame is better for competition. A heavier gun has less muzzle rise than the same thing with a plastic frame.
Doing better in IPSC/USPSA is (in my opinion) what is driving all these new steel framed models (e.g. the Walther Q5 SF).
If they bring out a Rival with a steel frame - and the price is still reasonable - I will definitely get one and relegate my current Rival to backup duty (for shooting USPSA and Steel Challenge and maybe use it in IDPA, too).