I reckon montreal would be the better option as well, althou you never know where he ends up.
MTL and BUF are both good options for him if he wants to be close to Haliburton. I haven't taken a great look at Buffalo's cap situation, so I can't comment on how it all fits together for them.
As a Sens fan, I really want him to stay in CBJ so we can get that extra pick.
On paper, if I am CBJ, would I trade a first round pick for Matt Duchene signed long term? Sure, of course - but if you get into the prospect of that pick being an unprotected 2020 pick after they potentially lose Panarin and Bobrovsky, I'm not sure if I'd do it in their shoes. I'm not saying I think CBJ will be bad, but they are entering uncharted territory where it will be really hard to project outward and guarantee that they'll make the playoffs.
Duchene isn't stupid, he knows that CBJ isn't the same team if Panarin and Bob leave, so I think it'll be important for them to show him they are going to make a splash by acquiring a goalie and maybe another player at the draft or in free agency. If Lehner decides to weigh offers on July 1st, he might be a really good target for them. If they can somehow get Hoffman in a deal with FLA for the 8th year of Panarin/Bob, that would also be another great move since Hoffman and Duchene had fantastic chemistry in Ottawa. Those are very long shot propositions though.