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Both (to my knowledge) are actually wood framed windows that have cladding on the outside.
In actuality, vinyl conducts heat/cold more than does vinyl.
This means that vinyl will lose heat/conduct cold more faster than aluminum.
But with a wood framed interior, such conductivity is neglible.
In short: it doesn't much matter.
But between the two companies, Andersen is hands down the superior product and company overall. Better millwork, better glass quality, better warrantee backup and replacement, better fit of parts...
The only way Pella excells is in options available