What it boils down to: if you don't wanna have to worry about it, spend a little extra for a 9". The 8" might make it, it might not.

 

9" is generally prefed for Classic Mustang performance. Advantages are that it is stronger, more gear ratios available, and a lot of the good aftermarket suspension setups are designed to work on 9" axle housings. The 8 will get the job done in most cases, or last for a while till you get a 9(unless it's some crazy engine, or launching too hard on slicks etc). Most people underestimate the strength of an 8" though.