For a roof that slants and faces north, which would give more light? a 24" solatube or similar tubular design or an actual skylight of similar size 20"x20"?
Given the interior reflective surfaces on the Solatube, they tend to reflect and bring in more daylight.
Without really painting the skylight shaft in a reflective, gloss, white, the solatube should give you more light.
I agree with the above comment. The nice thing about a Solatube is that you can angle the tube. So if it is possible to angle to the dome to a south facing roof line, you can capture better light. Solatubes bottom tube and top tube have 'angle adapters' which give you flexibility in where you place the tube. With skylights you are generally limited to straight up.
If you cannot angle the tube, another option may be to raise the dome up on the roof using a turret extension. Turrets extensions can go as high as is aesthetically pleasing. We have raised domes up as high as 8' feet on the roof. We do this to capture better light on exposures that limit the amount of light collected.
In closing, as the 'windowsonwashington' commented, the reflective material used by the Solatube and the dome technology should ensure more light than a traditonal skylight.
If you have any questions, you can email me at email@example.com or call 800-846-6621.
John @ Solabrite
Here is a link that might be useful: SolatubebySolabrite