Vintage set building question(50s-70s sets)

When building a vintage set do you prefer to sell your duplicates or try to build another complete set of lesser grade from the dupes? Financially& collection wise which method have you found the most beneficial?
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<< <i>You should keep the grades fairly consistent in the set. Don't put EX and Mint cards together >>
The exception is the high value cards. Adding higher grades in those cards to a low- or mid-grade set will usually be a good investement when you sell
Complete sets can definitely sell for a nice premium, since you are selling the work of putting it together in addition to the product
That way I can use those funds and buy more cards to complete the set.
Steve