For a Western Digital Duo, what's the difference between creating a RAID using WD Drive Utilities vs. OS X (diskutil)?
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.