I find it hard to believe that it took 4 years for someone to answer using the damage formula. Anyway, this is the formula.

damage=(level*2/5+2)*base power*attack stat/(50*defense stat)+2

source

Because everything but the base power and the used attack stat stay the same, the formula can be simplified.

what we care about=base power*attack stat

The base power is easy. It's 95 for thunderbolt and 65 for thunder fang. Figuring out its stats is a bit trickier. Luxray is usually a tank (source), which means it usually maxes the attack stat that it uses. If it were using thunder fang, then it would have max physical attack, which comes out to be 372 for Luxray. If it were using thunderbolt, then its special attack would be maxed, and that would make the stat equal 317. Now for the calculations.

65*372=24180

95*317=30115

24180<30115

Therefore, thunderbolt would be stronger.