On a three turns basis, a Toxic Orb holder will take less damage than a Flame Orb holder. But from fourth turn and on, Toxic Orb deals more damage since bad poison's damage worsens gradually. However, since Guts users like Heracross, Swellow, Luxray, Flareon, even Machamp or Raticate, aren't going to live very long before fainting, Toxic Orb is the better pick to provide them with slightly more staying power, although the difference is really meagre and can be ignored completely.
On bulkier Guts users such as Conkeldurr, Hariyama or Ursaring, Flame Orb is advised.
I hope this helps you.