I'm sure you'd need a wind tunnel to really nail it down however...
The aero profile of the wheel + tire is going to be determined by the tire's section dimension - the first # in the tire size. Both your wheel sets are going to be narrower than the tire's section dimension since the sidewall bulges out from the rim.
Based on that on piece of information only, I'd say the lighter wheel would win. However if it's aerodynamically very busy, and the wheel covers on the steelies are basically flat and smooth, then the steelies might be a better choice.
Of course steelies have virtually no bling appeal. Paint wobbly concentric rings on them to confound your observers.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
When I bought my xB I went from steel to Enkei aluminum wheels. Saved right on 8 pounds per wheel. Kept the original 15" diameter. So my xB went from 2415# to 2383# and I reduced the rotating mass and the unsprung weight. All winning moves. The only think I wish is that the wheels were a bit more aerodynamic. I'm thinking of Moon discs, but that blasphemes the Enkeis.
I'd vote for aluminum wheels--if indeed they're lighter. Use a scale. Larger diameters may weigh more than steel.