aluminium is just the finish, it's not solid aluminium framing.
This is what i'll get, it's dutch text, but you can see the diagrams and whatnot.
http://www.lein.be/pdfbestand/nl/nl_reynaers/nl_cs68.pdf
The alternatives are wood frame, maintenance
or a similar frame with PVC finish, which is a bit cheap looking, and turns dull after some years...
The aluminium is anodized, then powdercoated.. very durable and looks nice too.
Thermally, it's very well insulated, if it's 20 C inside the house, and 0C outside.. the inside of the window should be no less then 17C.
I could only get more if i went with triple glazing, but that would not be cost effective, since that would assume the rest of the house is a newly built, modern insulated house, close to a passive house (one that does not even need heating)
My house is from 1923, so if i were to pay for the triple glazing , i'de loose the bit i gained in the window, trough the wall instead...