Hard to say.
Kenobi is master at the defensive style but he seems too passive in his fights at times, and waits to tire out his opponents. This will not work with a cyborg like the good general.
On the other hand, Grievous is well-equipped to overwhelm defences of someone of Kenobi's caliber, given his mechanical, spinning arms and amazing striking speed (about 20 strikes per second that will surely overwhelm Kenobi's Soresu)
As of RotS I think the general has a clear advantage. Grievous wins this 10/10.