Steel vs Aluminum windows

I've heard that as well , but haven't seen any documents to verify it

Fundamental difference is whether you are looking for a fire-rated window or not. If the city is not going to inspect and DOB approval is not needed, you may not decide on fire rated windows. If you want to comply with all codes, this is a little bit of a murky area (I recently researched it), I would go with Fire-rated windows. These windows need a special glass and special framing. A companyy in bronx makes them.

http://capitolfireproofdoor.com/fire-rated-windows/

We replaced all of our windows last year. The lot line windows came from Optimum. Looking at them, I would have thought that they were aluminum, but perhaps not. Sadly, they were quite expensive, and are not remotely energy efficient like the Marvin tilt and turn windows installed everywhere else. They also have wire mesh embedded in the glass -- you can get clear ceramic fire resistant glass that is just silly expensive and come in limited sizes.