It's not that easy choice, Lucic is a special player, but I'd take Letang...
Lucic doesn't change how we play. We need to play better hockey. Sure, MLs intangibles would be tremendously valuable, but we would still not have a game.
People complain on our depth players, but that's because we don't have a game. Its no coincidence that guys like Nolan and King for LA and Bickell for Chicago gets a ton of attention, but these guys would definitely not even get a fraction of that had they played for like Buffalo.
A great team can ways find guys like Bickell, King and Nolan cheap. You won't find a right shooting PPQB cheap...
Also, yes Letang is a product of Pittsburgh. But that does not mean that his abilit is temporary, due to playing with the two greats he has developed his game. However, he is also overrated somewhat. But he is a very good D.
But how good is Lucic really? Under Torts, I am fairly certain that someone like Lucic could easily have lost 20+ pts. Boston got a transition game and gets up I've thanks to people other than Lucic. Lucic is only effective up ice.
So a lot of what some say of Letang also applies to Lucic.