Flame Orb, definitely.
Why give your Luxray toxic poison, costing your Luxray more damage each turn, when you can activate Guts with the burn condition that only takes away 6% of your HP each turn?
Hope I helped
I actually disagree with this. First off burns deal 12% each turn, not 6%. Toxic poison, however, does start at 6% and starts to stack up after that. So on the first turn and second turn you're in battle, toxic poison saves you more HP than a burn does. On the third turn they're tied, and after that burn saves you more HP.
So really it depends in how long you want to keep Luxray in play. Generally your opponent will force Luxray to switch out within four turns, so I'd honestly choose Toxic Orb. They both activate Guts so they're both good options though.
commented 3 minutes ago by ƒιzz
This an excellent point made by ƒιzz, especially because in competitive you don't really keep one Pokemon in that long, so Toxic Orb may be better. Really though it is up to you.