I like them both, but I chose Iris. (I'll talk about Alder in a minute.)
For Iris, they gave her some solid development with the Memory Link, and while her character wasn't very frequent in the game, I still thought it was kinda nice. And for battling her, not only is she the only battle in the DS era, or any pre-XY game for that matter, to use full body VS sprites, but she has that outstanding multicolor beaming battle background, and that battle theme, oh my fricken Jesus in high saint's heck, her battle theme, which I can never get enough of. She also uses several of my favorite Pokémon, which is why I always have such a fun time battling her.
Alder is great too, mostly for the fact that he actually steps up to try to fight the big villain, and he too has some depth. While he wasn't as fun to battle as Iris, I still liked him for what they had to give him.