My vote would be for Zenyatta. I personally like her more and have seen more of her races, which makes me rather biased honestly. But I think the way she raced and held back so much before letting go would have put her ahead.
The only way Rachael had a chance to beat Zenyatta is if the match race had been run in Rachaels 3 yr old campaign. As a four year old Rachael was not the same horse, hence her retirement so quickly .
But, I still believe Zenyatta would have won, she is just a magnificent race horse.