I would trade Hall for the 1st pick in this year's draft. I wouldn't trade him for the 2nd, and I certainly wouldn't trade Hall+ for the 2nd.
Hall is 23 years old and is already arguably the best player in the league at his position. Any rebuild (re-rebuild?) that would see McDavid as the centerpiece would still be best served with Hall as a major part of it.
If McDavid is really the next franchise-changing prospect ala Crosby as he's been touted as since as far back as I can recall, then we'll want to surround him with more players in the current core's age range than McDavid's age group.
The Penguins were a lottery team in Crosby's rookie year, a playoff team when he was a sophomore, and were contenders by his 3rd year. In McDavid's 3rd year, our current "core" group would be Hall-26, Nuge-25, Yakupov-24, Eberle-27, Schultz-27, Draisaitl-22. That's still a group in the infancy of their prime years. There's no reason to push that cluster back farther by moving out Hall for a player who, in all likelihood, wouldn't be as good as him at that time.