Base Stats are all average, with the average being 83.3 and its best stat being Special Attack.
Has access to fairly strong moves, such as Ice Beam, Hydro Pump, and Psychic.
Has a total of two weaknesses, Grass and Electric, Grass of which can be covered by Ice Beam.
Has an amazing Special Attack and a decent Physical Attack, making it a worthy mixed attacker.
Has access to great moves, such as Psychic, Poison Gas, Psych Up, and more.
Can learn a move to counter every one of its Weaknesses
Both of these Pokémon are great for a Crystal Playthrough, but based on the overall potential of both of these Pokémon, I would choose Golduck, but Hypno is not a bad choice at all. Good luck!