Definitely Swampert.
Charizard is 4x weak to Rock and 2x weak to Electric and Water and can only cover two of those weaknesses with Earthquake. Swampert is only 4x to Grass and covers that weakness with Ice Beam.
Charizard is 4x strong to Bug and Grass, 2x strong to Fighting, Fire, Steel, and immune to Ground. Swampert is 2x strong to Fire, Poison, Rock, Steel, and immune to Electric.
Charizard has STAB Fire and Flying moves. Swampert has STAB Water and Ground moves.
Charizard has higher base SAtk and Spd. Swampert has higher base HP, Atk, Def, SDef, and total stats.
Charizard can breed in the Monster and Dragon Egg groups. Swampert can breed in the Monster and Water 1 Egg groups.
Close in some areas, but not really in others.