Poison Jab has horrible coverage with your current set. It only has the ability to hit Grass type Pokemon, but Poison does have decent neutral coverage. However, for in game purposes, coverage is one of the most essential factors in progressing. Aerial Ace allows you to cover Fighting and Bug, two types you currently do not have covered. Poison Jab only covers Grass, with Bug covers.
So its really your preference - decent neutral coverage but bad type coverage, or average neutral coverage and good type coverage.
