Irving is more of a threat because Lebron is one of the few guys they have that can defend well so he has to expend more energy at the defensive end.
Plus, Lebron is more efficient and will get his points.
If you make life hard on Irving he's more likely to finish with a 6 of 20 night than Lebron is and that would be an almost automatic win seeing as how Cleveland has to outscore good teams to win.
Plus, you pressure the guy with 2 rings or the guy who has yet to play a playoff game.