Reuniclus is likely the better option offensively, since it has Recover to shrug off lost damage if it takes hits rather well.
It is rather tanky, given it's massive HP and moderate bulk. One of it's main Abilities, Magic Guard, can be profoundly useful in various situations, since it protects it from indirect damage.
It can also learn Focus Blast to get around Steel, Ice, Rock, Normal, and Dark Types, though it is situational at best.