Where material sits matters more than how much there is
Two beams with the same outer dimensions and very different amounts of material can have similar bending stiffness, because material near a beam's neutral axis contributes far less to resisting bending than material placed away from it. This is the entire logic behind an I-beam: most of the material sits in the flanges, far from center, where it does the most work. A solid rectangle uses material everywhere, including the low-value area near the center, making it the heaviest way to reach a given stiffness at a fixed outer envelope.
How the comparison works
Each section shape carries a relative stiffness-per-mass multiplier versus a solid rectangle at the same bounding height and width: an I-beam-like profile and a hollow box both outperform a solid rectangle meaningfully, an L-bracket profile more modestly. The two shapes you select are compared directly, with the more material-efficient one presented as the best fit and the other as the alternative for when print simplicity matters more than efficiency.
Worked example
A solid rectangle versus an I-beam-like profile, both at a 20 x 20 mm bounding envelope: the I-beam-like profile offers roughly 2.4x the stiffness-per-mass of the solid rectangle, at the cost of needing internal geometry that is more work to slice and support cleanly than a plain solid block.
A common mistake
A frequent error is assuming a material-efficient section like an I-beam or hollow box automatically translates to a printable one. Thin internal walls that look fine in a CAD model can fall below your printer's practical minimum wall thickness, or require internal supports that defeat the whole purpose of a hollow design. Check the internal geometry against the Minimum Wall Thickness Check before committing to an efficient section shape.
Limitations
This uses relative moment-of-inertia logic to rank shapes at the same bounding envelope, not an exact computed stiffness value for your specific material and print orientation. FDM's anisotropic layer strength means a shape's real-world benefit depends on how it is printed, not just its cross-section; validate with a real test part.