Raccoons have higher attack, speed, intelligence, and health stats. Opossums do have better defense, but lack stamina and rely on their slight luck advantage.
Raccoons are the superior starter, they are the Charmander of the options.
So, the Raccoon will initially seem very good until the first gym where he gets 1-shot by Onyx? Oh. That doesn't seem so good at all. I guess we should have chosen the Squirtle analogue.
Bulbasaur absolutely mauls the first 4 gyms, and there are plenty of water pokemon available at all stages of the game which are capable of trashing your rivals Charmander/Charmeleon/Charizard. The ice and fire types that are most effective against grass types are buried later in the game.
And the it still suffers greatly by the second gym, and the final evolution isn't even a real dragon, so it gets absolutely wrecked by rocks, but I still never learned my lesson because it's cool as hell.
Fair enough. I might be sightly biased as well because when playing the originals, I had Blue while my best friend at the time had Red, we each chose the respective starters, and I've just stuck with that and taken the water starter in every game since, despite them often being a sub-optimal choice. That said, Mudkip is one of if not the strongest starter choice in any pokemon game, so it has served me well enough.