Spare yourself the trouble of using Dustox in a legitimate playthrough of Platinum. Bug/Poison as offensive types, as well as lower offensive stats than the unevolved starter Pokémon, forces it to use Toxic + Moonlight to successfully defeat an opponent's Pokémon, which is far too resource heavy and time consuming to be effective. It takes until level 34 to make use of its Bug STAB, and the TMs for Shadow Ball and Psychic are not well-suited for it. Its sole merits are an alright enough Sp. Def stat, as well as its resistances allowing it to wall Gardenia's team and dish out fine enough damage with Gust.
Lopunny serves passably with good Speed and below-average Attack stats. Its obvious detriment is being a Friendship evolution, needing to go from 0 to 220 Friendship to evolve. This would be met really late in a playthrough if you do not go out of your way to farm Friendship for your Buneary, making doing so essentially a requirement. Lopunny's most defining trait is that it gets Return at level 13 through the Move Reminder, allowing it to not waste the TM. It also gets Frustration, which could very well be at 102 BP when its first caught.
Neither Pokémon are any good for a Platinum Nuzlocke, especially in comparison to Pokémon you are guaranteed to access (Gyarados, Jolteon, Haunter/Gengar, etc.), though Lopunny is far superior if you are willing to put in the effort necessary to evolve it.