Poll
29%
71%
I'm not sure what form Obi-Wan uses but doesn't Bane say
"You will sacrifice reach and leverage but you will be able to create an inpenetrable shield of defense" and "Your opponents must expend precious energy with each blow, while you remain fresh and strong"
Source-ROT-P174
She also manages to fend off an Echani Weapon Master's apprentice who is said to be a Jedi Knight just as Zannah is still an apprentice.
I believe Zannah would win this fight unless Obi-Wan can pull something out of the bag.
Blaxican is right about Obi-Wan being the best practitioner of Soresu so it'd be difficult to call on lightsaber. Zannah's mastery of Sith Sorcerey would play a factor in a Force battle, but Obi-Wan has experience fighting Sith Lords. All Out I would have to give it Obi-Wan - so I'm refraining of the poll.
Given the nature of Soresu I see the fight going like this:
Kenobi and Zannah stand several metres away from each other. Each one waits for the other to attack.
And waits.
And waits.
Kenobi: Well come on and attack me!
Zannah: No, you attack first!
Kenobi: No, you attack first!
After a few minutes of this Zannah says "oh ***** this!" and uses Sith sorcery to make Kenobi insane.