For a Western Digital Duo drive, is there a major difference between the two? OS X Disk Utility doesn't have the RAID feature now, but OS X can still create RAIDs via the command line and diskutil. WD Drive Utilities can modify RAID configuration, erase drives, etc. If formatting a RAID, is there any reason to choose one method over the other? (Namely, performance and stability.)
There is in fact, a major difference between the two.
The WD Duo has hardware RAID built into the device whereas OS X RAID utility is software based RAID meaning you use CPU cycles to manage the RAID processing overhead.
The WD "offloads" the RAID processing overhead onto itself and presents just a drive to OS X. OS X doesn't have to do anything more than it already is doing to access it. The WD Drive Utilities is just a management tool for the RAID array, not the RAID software itself.