Indeed, Jessica seems to be more powerful but Anya is a reckless and stubborn fighter. She never gives up. Please describe the fight if you want. I think I will try to do something with your posts (Like this)
I still think Jessica has the upper hand. She is way stronger and she can take more damage. If she avoids Anya Storm's lightning and manages to get close, she could drain the life out of her or crush her skull. Anya would also have to deal with Jessica's minor demons. Granted, she could probably fry them, but her attention would be diverted. Also, Jessica can probably resist Anya's lightning but can Anya resist demonic energy?