Usually that difference in skin reflects a difference in construction. The aluminum siding is popular on "stick-built" structures, wood-stud walls a light weight version of how frame houses are built. The wall gets its strength from the framing.
When you see fiberglass skin, you are usually looking at laminated panel construction, an aluminum perimeter-framed foam core with skin glued to both sides, skin can be wood or other composites. The wall gets its strength from bond of skin to the core.
There are some exceptions, as Winnebago started out with aluminum outer skin when it developed the first laminated panels, and some builders like Northwood have put fiberglass skins on wood-framed walls. Then there are the high-end manufacturers (like Excel) who built an aluminum studded frame structure and cover it with with composites, or Holiday Rambler who used to build an aluminum studded frame and cover it with aluminum, then later switched to fiberglass skin.