I'm not saying that it is wrong to choose Gaudreau over Duchene on paper, but a lot of people in this thread are underrating what Duchene brings to the table and ignoring the context that might makes this trade a fair one for one swap.
Regarding Gaudreau being PPG, Duchene was PPG at the same age.
Duchene brings more outside of offense. He is good on faceoffs, can kill penalties and play both sides of the puck.
With all that said, I still take Gaudreau over Duchene, but then when you observe the context of the Flames and Gaudreau possibly being unable to agree to a contract, it makes the trade more even. CGY gets a cost controlled 25 year old 1st liner with PPG upside for Gaudreau. If they can resolve their differences and agree on a contract, or if someone out there offers them a more desirable one for one swap, obviously they don't do it.
The unfortunate thing is that I don't know if Colorado would do the trade if the requirement is that they sign Gaudreau to an 8M per contract. This is the same team who got rid of Ryan O'Reilly who is one of the best two way centres in the game, and arguably their best player at the time over similar contract demands. Maybe they consider Gaudreau a special enough player to do that for.