BDSP or S/S

The expansion pass in S/S let’s you catch the legendaries in the max lair

Both are great. Check the in game Pokémon since each version have their exclusive. The storyline would be the same. If you want dialga then go for BD. If you want palkia, go for sp. I might get BD for dialga but the exclusive in sp are great too. Check more here.
https://www.pockettactics.com/pokemon-brilliant-diamond-and-shining-pearl/exclusives

Fyi, you do talk about pokemons right?