It depends on what you want to play on it and how much it costs. Lower model Di Giorgio's only sound good for bossa nova but I have a 1970 Author 3 which has been refinished in french polish and it sounds fine for classical too.
The Fora De Serie is Di Giorgio's top model and supposedly sounds fine for both classical and bossa nova. If you just want to play classical I'd look for a '70's Yamaha GC series.
I think from a purely investment point of view an early Author 3 would sell more than a Fora De Serie because of it's association with Baden Powell.
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