>>23166090The GCN games are more challenging than main series games because the important character's teams usually have a strategy going on. You also have a smaller pool of Pokémon available, and in your usual run you'll end up with a team around the same level as the opponents, unlike the main series where people usually have overleveled Pokémon.
IMO, the story on Colosseum also is a bit better.
It's also interesting how Rui never denies when people comment on her and the MC looking like a couple, and even gets mad when an NPC tells you to ditch her.