i would use the spell you are emulating with wild shape and your wisdom modifier but i understand that this is very unclear. On could also argue that according to this FAQ (http://paizo.com/paizo/faq/v5748nruor1fm#v5748eaic9o6x) you use the level derived from the minimum level when you get the spell.
|James Risner Owner - D20 Hobbies|
It's pretty wild that however many years into Pathfinder we are, this fairly basic question about a core class's highlight feature remains unanswered.
We have two answers. It's the GM's job to pick between. The two answers are going to give the same answer (or very close) for single classed characters, so it shouldn't matter too much?