I take proven MLB players every time. And, to me, Harper still has incredible potential or upside.
I think this is ignoring that what Bryce has "proven" in 5 of 6 seasons is that he's going to go through a significant portion of the season slumping. People were complaining about Juan Soto having a 30-game stretch where he put up a .240 average with a .383 OBP and just didn't have any power coming with it. He OPSd .729 during those 30 games.
When Bryce was 19 he had a 55-game stretch where he hit .204/.273/.290. A third of the season, hitting like Christian Guzman. And just this year had another 41-game stretch almost as bad, hitting .175/.271/.357.
He's a temperamental guy, and when his timing goes off it stays off for a while. I'm not saying I wouldn't like to have both, but I certainly think if both guys are likely to have similar stats it's better for the baseball team to have more consistency and less boom/bust.