It depends, but in general, a sistrus berry is better. The main reason being that oran only restores a fixed 10 HP, which is generally quite awful on Pokemon with high HP. But Sistrus restores a percentage of HP. So it will always restore more on higher Hp Pokemon.
On low HP Pokemon however, sitrus may not be the right choice. If a Pokemon only has say 14 HP (in LC for example) then Sitrus is only going to restore 25% of that. Not a lot. But an Oran would restore way more.
hope I helped ^.^