Whether or not you would need
Drive Genius depends on what you already have in the way of disk utilities beyond DiskWarrior. If you've got
TechTool Pro, Drive Genius is less likely to be needed, even though it has some functionality not available in either DW and TTP. Without TTP DG might be considered if the price is right.
FWIW, I own copies of all three utilities, and DG is the only utility I never actually needed. I fired it up a few times, but the other two covered my needs well enough.