I find Grievous's feats towards the end of the Clone Wars more impressive, ie. smacking down Obi-Wan in those recently released TCW episodes with the incomplete graphics, dueling on relatively even ground with Maul in SOD, holding his own against Windu briefly, and almost overwhelming Obi-Wan's defenses with his offense in ROTS.
Plus, IIRC, most of the accolades Grievous has referring to his saber mastery are afforded to him around LOE/ROTS
I say Grievous, personally, if we take them both at their very best, which I don't think TCW does for Grievous.
I mean, you can make justifications for some of Grievous' showings in TCW but the fact is he's just put in some ridiculously underwhelming situations - although his showings against Kenobi are consistently impressive.