Let´s say you do not like the petty skylights. You want some nice, large windows to let the most possible sunshine under your roof. In that case you can choose from the roof windows or the dormer windows. Which ones are the best? It depends.
The roof windows vs skylights
The roof window is something like large skylight. Actually there is no substantial difference between skylights and roof windows, nevertheless skylights usually refer to smaller surface. Both skylights and roof windows are installed aslant, to the surface of the sloped roof.
The roof windows advantages
The roof window offers better (and longer) illumination compared to the vertical, dormer window. In certain areas the difference can be as much as 40%.
As the slant roof windows are installed directly to the roof surface, no special frameworks are necessary and installation of the roof window is cheaper than the dormer window installation (less material, less work).
The roof windows disadvantages
The roof windows do not add any space to your loft or ceiling as they only copy the roof.
Also the view from the roof window is little limited compared to the dormer window view.
The dormer windows
The dormer windows refer to the vertically installed windows projecting out, from the sloped roof surface.
The dormer windows advantages
Dormer windows increase the space of your loft or ceiling.
The vertical viewing (dormer) window also offers better view as you are closer to the viewed object.
Last but not least, the dormer windows, as they lift the sloped part of the roof higher, improve the safety conditions of your ceiling or loft (means you will not hit your head when looking out of the dormer window, of course, if you comply with all standard recommendations for the dormer window placement and dimensions.
The dormer windows disadvantages
Unlike the slant roof window, the dormer window faces the direct daylight with smaller surface, therefor it lets less light in.
Also the dormer windows installation is usually more expensive and demanding on planning and sources than the slant roof window installation.