IIRC, the Platinum E4 goes up in level faster as you rematch them repeatedly; Lance's highest-leveled Pokémon in the rematches is level 75, whereas Cynthia's is level 78. Cynthia's Pokémon go up in level if you beat her often enough, though; I'm not sure if Lance's do, but I'd expect him to stay behind Cynthia if you max both of them out in rematch experience.Platinum E4? Why not the SS E4? Considering they're higher level.
A bigger consideration, btw, is that you may want to train Pokémon from Platinum on SoulSilver and vice versa, for the 1.5x experience bonus for winning with a traded Pokémon.