I much prefer Sableye & Mawile for those reasons :
1/ They are different from each other (in terms of type and moveset)
2/ They don't look like lesser clones of the franchise's mascot
3/ Sableye is actually usable enough, thanks to its hidden ability. Also in Sapphire, it greatly helps against Brawly, the second gym leader in this game.
While they don't win in the cuteness department, this isn't enough for me to prefer using the cheerleader duet, as I call Plusle & Minun.
Hmm, let's see. Sableye is a neat Ghost/Dark Pokémon and Sableye is an awesome Futakuchi-Onna based Pokémon. Plusle and Minun, on other hand, are both boring Pikachu clones with uninteresting gimmick abilities. I really can't choose here.