Flat Window or Cupula on Flat Roof Extension?

[greido]

I've eventually started my extension in Edinburgh of my 1930s detached bungalow, the wall between my existing living room and Dining room will be removed, as wil the external wall and a relatively large extension will be added to create an open plan kitchen/dining/living space.

The new extension section will be approx. 10m x 5.5m, there will be a flat roof on this section with 7.5m sliding doors and two 2m x1m skylights. I had intended having flat glass windows on the roof but both the builder and roofer have both, separately, recommended that I get Cupula windows instead as they will be easier to keep clean and also give an illusion of additional head-height. 

Has anyone had any experience of Cupulas v flat windows? - If so, what would you recommend and why?

Many thanks in advance.