I'm gonna be in the minority and say Wallace is the better Champion.
If you didn't pick Treecko (which at this point I never would), good Grass-types and Electric-types to counter his team are hard to come by in Hoenn, and that's what makes battling him so difficult. You're gonna need some strategy and a lot of extra firepower to take down the monstrous tanks that are his Milotic, Tentacruel and Whiscash - even in the remakes, where he battles you in the post-game with the same team as in Emerald! He gave me hell in my playthrough of Emerald (which, ironically, only intensified my love for him.) Steven is actually really easy due to the early abundance of Water-, Fire- and Fighting-types, and if you picked Swampert like I did, he might as well hang the towel before the battle even starts. The only scenario where I can actually see Steven being hard is on a Mono-Ice run because that type is weak to both of his preferred types, and I'm unlikely to ever do one of those because it's my least favourite type.
I've always felt like Wallace was more representative of the region's traditions, too. Not only does he train Water-types in a region that can be summed up as "7.8/10 too much water," but he also belongs to one of the region's original communities, the Sootopolitans. As if that weren't enough reason to like him better, in addition to being an accomplished battler, he is also a Master Rank Coordinator in the very region that introduced Contests, as well as the mentor to his niece Lisia. Lastly... I was going to put my older man bias aside, but it's extremely relevant in this case. One of the reasons I came to love him so much is that he just gives off a more mature, experienced vibe than the young and often arrogant Steven does. Heck, the Delta Episode only cemented that fact for me, because I thought Wallace was the only genuinely helpful person in the entire quest while Steven just left me baffled and kind of pissed with his impulsive behavior.
So yeah, Team Wallace forever. Go home to your rock collection, Steven!