Obi-Wan would win against Mace.
There is no way to defeat a Soresu master like Obi-Wan in a lightsaber duel because you won't get a chance to hit him. If you read the RotS book written by Matthew Stover you'll find some lines that state Obi-Wan might be the best lightsaber duelist alive.
Mace himself says:
- that Obi-Wan might be the only Jedi who can defeat General Grievous
- that Obi-Wan is the "true" master of lightsaber combat because:
a) he is the master of the "classical" fighting style
b) he has no weakness that he has to compensate by using a certain combatstyle (Yoda uses Ataro because of his lack of range and Mace uses Vaapad to use his "darkness" as a weapon of light)
So Obi-Wan would win against Mace.
I'm undecided myself. I don't exactly put too much into the ROTS book, because the novelisations tend to glamourize the movie characters a lot. But after reading Shatterpoint, I find Mace to be every bit as cunning and wise as Obi if not moreso. This won't be a straightforward match. I believe it can go either way.