Live axle pros: Stronger, ability to have instant center, very large
Cons: inability for each rear tire to track independantly
Independant pros: ability for each wheel to track independantly, smoother ride, less unsprung weight, generally takes up less room
Cons: more fragile, very little instant center(25% max as opposed to 100%+ with live axle), more expensive, more comples to work on, more difficulty getting power to the ground.
If I were to choose one it would be live axle. You can make a live axle work almost as well as an IRS, but it will always be stronger and have more available traction.
However, if you were strictly building a competition only road race Impala, IRS is the way to go.