Poison Jab is better. Seviper shouldn't be trying to induce status since its defenses don't really allow it, so more power is the way to go and Poison Fang is just far too low in power. The increased chance to Toxic-poison is nowhere near worth it.
If life gives you lemons, sell 'em to Tom Nook. Black 2 FC: 0132-9980-0476, 3DS FC: 5284-2520-2972 (Pokemon IGN: Aki)