You want Drain Punch. While Toxtricity has a higher Special Attack, its Attack is perfectly serviceable as well, and Tox can be mixed easily. As you said, you don't have a lot of Fighting options, so Drain Punch is all the better. Fighting is great against Gordie/Melony, Piers, Rose, and a lot of random Pokemon throughout, while Boomburst is just a high-powered move that's neutral at best.
This isn't to say Boomburst is bad -- you do have other options for Fighting moves, like Togekiss' Aura Sphere and Dracovish's Low Kick. I do, however, think that Toxtricity benefits more from Drain Punch.
Hope I helped!
