I wouldn't be surprised if Ash does beat Iris (I personally want a draw but with how the episode is worded and showcased unfortunately there will be one victor). My thinking is simple here with Iris officially becoming Unova's champion as confirmed via the official account for the Anime on Twitter. If Ash is going to be battling Elite 4 members, Champions. He's going to need to beat them all to face Leon. (I'm going to galaxy brain below so apologies)
With Iris, if she does happen to lose, she gets to go on a little revenge arc. As between Ash and Iris there was a lot of friendly competition. The only other character that can fall in this spot was Misty. (May, Dawn and Serena being distinct contrasts with different goals being Pokemon Coordinators). Iris besting Ash in official competition with her victory in the Anime Exclusive Club Battle Tournament held in Nimbasa City. While Ash got revenge during the Pokemon World Tournament Junior Cup (Albeit with Iris' Dragonite being disobedient). A victory is still a victory.
Because of this due to history Iris is technically speaking Ash's best inidividual rival because of her re-appearance. Both are respective champions, (Unova - Iris, Alola - Ash). So going into this presumptive 2v2, I'm going on record to say that Ash is the favourite not because of battling abilities, it's to rectify the faux win that Ash got during the Junior Cup arc.