First turn, Toxic Orb does 6% damage, and stacks up. Flame Orb does 12% on the first turn. That means for the first two turns, Toxic Orb saves you more HP, third turn their tied, and then Flame Orb saves you more. You'll most likely not be staying in for that long in competitive, especially with a set like that. So really, Toxic Orb is better. Something I learned from another question.