Hollow bars

Does the torsional stiffness equation apply when comparing a solid and hollow bar or two hollow bars of different inner diameters?

I assume you are talking about anti-roll bars. But not sure where did you get equation? Post it here, so we know what we are talking about.

But generally:

1. If you measured bar stiffness (in practice) you can use it in spreadsheet

2. The solid bar (or the one with thicker wall) is stiffer than one with thinner wall (assuming outer diameter is equal).