I would prefer to use copper, inside buildings, it is not only the cost of the cable, it is its size and the cable conduit, for example a THW cable ( Without derating, 30ºC ):
A Copper Wire, THW, size #8 have a permissible current capacity of 50 Amp.
Aluminium Wire THW, size #8 have a permissible current capacity of 40 Amp.
So for 50 Amp you would need a #6 THW AL conductor, so all the installation would be bigger.
So for a building I would use Copper, also it is wisely to be sure depending in which country are you working that the contractors have enough experience with that type of materials, and knowledge of the appropriate tools and hardware required for that job.
For example, in Latin America some country's have several importation problems so waiting for non-standard hardware and tools would delay the project. So what you think is cheap would cost more at the end with unpredictable results due to the lack of experience of the contractors.
Hope that someone with more experience using AL wires gives us a good example an a real life example using AL wires. I would like to improve my knowledge in AL cables for this type of installations.