Zeke - what is being done today in place of capping with aluminum? My neighbor had new windows installed last year and the contractor capped the old wood with aluminum.
STRENGTH - folks the window is not a structural member of your house. It just holds the glass in place. A reasonable quality vinyl made by a known manufacturer will be fine. No one really cares if the replacement windows are fiberglass or "CARBON FIBER"
They do care if they are replaced for cleaning and insulation matters.