They do the same thing, but they're different elements. The point of having the both is to have elemental diversity so if you run into something with high earth resist, you use Jabs, and if you run into something with high air resist you use Shaker.
Shaker has the highest DPT in the end, but less hits. If you don't have 100 Power and are against an enemy with near-equal air and earth resists, use Jabs to build up your Power faster. If you're against an enemy with near-equal air and earth resists and you have 100 Power, use Shaker.
Some enemies will sometimes be reactionary towards attacks, whether negative or positive. It may call for you to use cheaper attacks plentifully, or more expensive attacks sparingly. The differentiating AP costs are for that as well.