I think there is no way the team chooses Harper over Rendon. They will not sign Harper unless they have Rendon locked up beforehand. Once they do, then they can evaluate whether they can afford Harper. Until they know how much Rendon will cost they cannot evaluate that.
And despite popular theory ("he's going to the Cubs", "he's going to the Dodgers", "Phillies") nobody has any freaking idea what he's thinking. After signing Rendon and evaluating what's left for Harper, the team may say "take it or leave it". He just might take it. Especially when he finds out that nobody is offering him that $4M contract and the best offer is $210M for 8 years, and he wants to be in DC anyway.