It’s a battle of the young guns vs. the old reliables in the first MLS is Back Tournament semifinal as the Portland Timbers and Philadelphia Union will meet to decide which team will grab the first spot in the final on Wednesday. The Union have been buoyed by academy stars Brenden Aaronson and Mark McKenzie — in addition to some superb finishing from Sergio Santos and veteran leadership from Alejandro Bedoya — in their run to the last four.Portland's MVP has been 32-year-old Sebastian Blanco, who has two goals and five assists in the tournament, while Timbers legends Diego Valeri and Diego Chara are still going strong, even with Valeri shifted to a bench role for the quarterfinal. But don’t let anyone tell you Portland are lacking youth, as 23-year-olds Jeremy Ebobisse and Eryk Williamson have broken out in Orlando.