The burn rate for 2400 and AA#9 is similar. The main difference between the powders is that AA9 is a ball powder that is relatively dense, ie. it takes up less volume in the case per grain of powder when compared to 2400 that is a flake type powder.
Of the two 2400 is perhaps a bit more versatile than AA9 and being a relatively old powder you will find data for more cartridges than AA9, esp if load lead bullets for a rifle.
I load both the .357 mag and .41 mag using 2400 and AA9. I find velocities are often faster than the book indicates with 2400 while H110 and W296 generally produce near predicted velocities. AA9 generates loads similar to 2400 with aproximately 1.0 grain less powder in the .357 mag.