Is the Microblaze soft cpu better than the Cortex M3 soft cpu
The two major points are:


The Microblaze is a well supported soft core.  Many other IP designs are made to interface with it.  ARM is popular, but you'll have less support available, especially from Xilinx, who designed the Microblaze.
Using an ARM core will let you use compilers (and code) designed for the ARM architecture, which is desirable because (according to my totally unbased guess) more code is written for ARM than for Microblaze.


I'm not familiar enough with either processor to make further comparisons without a list of priorities.  What do you value in this processor?