About comparing... you can build thousands of petalinuxes with different configurations. Also, you can have hundreds of Microblazes depending on how do you configure them, not to say their performance will also depend on whether they run on Artix -1 or Virtex -3. There are as well many types of "ARM" processors. So your question is kind of "are Asians more clever than Europeans?" - close to nonsense, isn't it? Also, there are many ways to measure performance. What exactly is "performance"?.
In general, Petalinux will perform better on an ARM processor than in any Microblaze. But there is the possibility that the crappiest ARM core (I'm thinking of a soft IP) will be worse than a full-fledged microblaze.