Hall is a much better player than Duchene
It's a lot closer than you think.
For one, the value of a Center versus a Winger is going to close whatever gap there is a decent chunk.
Also, Duchene when he was traded from Ottawa as a rental was on a torrid 45 goal and 95+ point pace that season. He actually had 27 goals and 58 points in 50 games for Ottawa before he was dealt. Columbus without questions believed they were getting a bonafide #1C in return when they gave up the assets they did to get Duchene.
Hall on the flip side is having a down year this year with just 4 goals and 21 points in 24 games, so far*
Especially as free agents, they will return very similar packages. History has shown time and time again that big time pending free agents always get underwhelming returns because the market value difference just isn't extreme when you're talking about players who you potentially only have for one playoff run and they're gone again.
A 1st + B level prospect or two will be the return, with a conditional pick added if Hall signs with the team he is traded to.