For aesthetics, I'd go with wood.
If you use redwood, hand select lumber that contains the 'red' wood and not the 'blond' redwood. Now days much of the big box stores sell redwood with very little 'red' wood and so it isn't resistant to insects or rot. It works OK for a treated deck but won't last long in a humid, wet greenhouse.
Having said that, my greenhouse is galvanized metal
Steve