The thing that the hollow, injected irons allow is a thinner face design, that can (on the lower lofted irons) create a very slight increase in ball speed. The injected material in hollow designs improves the sound and the feel and is needed on thin face designs. That being said, I think it would be difficult for most players to notice much difference in performance between the two IF the lofts were identical. Really good ball strikers with higher clubhead speed might see a few yards difference in the lower lofted irons. The more loft, the less effect a “thin” face will have because the strike is more of a glancing blow. Hollow, thin face designs do give us designers more discretionary weight to use to increase the stability. Even a few grams saved that can be placed strategically in the perimeter, or the toe area or the sole area can benefit the playability. That being said, you can still achieve a high degree of playability in solid forged designs as well. This is accomplished by simply being aware of the mass and dimensional characteristics and distributing the weight properly.
Britt Lindsey