I am not a huge fan of the third Generation of Pokémon. Why am I here? Probably because I feel the need to smell you later. Anyways, I felt that both Wallace and Steven were too fond of their Pokémon Types being Steel and Water.
Having great Electric Pokémon available means doom for Wallce.
On Steven: If you're smart, you grabbed a Pokémon with Flamethrower.
I try balancing all of My Pokémon teams and trying not to have more than one over lapping type. But, in terms of difficulty and weaknesses?
This goes to Wallace. Wallace is the better than Steven, albeit by not very much. You can absolutely destroy Steven's Champion team with a single Pokémon knowing a good Fire Type move and any move type that can one shot his Claydol (Bug, Ghost, Dark, Water, Grass or Ice), but Wallace has that awful Whiscash that always seems to miraculously survive more than three turns (Unless your Electric Type knows a Grass move, you'll be switching out quickly).
I know people like Steven, but Wallace's Whiscash trumps Steven's Claydol in terms of weaknesses. Whiscash is only weak to Grass type moves (x4), while Claydol is weak to six different Move Types (x2), giving you loads more options at dealing with Steven's team.
Both Champions, to me, are weak sauce.
Probably should mention I don't have the remakes.