Who says you can't use both! They have no type overlaps, and Malamar is a great Pokémon to use in-game. However, according to this tier list, Buneary is ranked C, while Inkay is ranked B. So Inkay is probably the best choice.
Inkay also has super-effective STAB on Hala, the first island kahuna. Since in the early game you don't get a lot of options for Pokémon, (usually Route 1 Pokemon have Normal types, which are weak to Fighting, which isn't quite good.) super-effectiveness is great for early game.
Inkay also has a fine level-up movepool, learning Foul Play at level 8. Also, the elemental punches aren't that useful for Lopunny. However, just choose what you prefer if you have one more slot and you don't want to replace any Pokémon.
Inkay is still better statistically though.
