Does OS X's Disk Utility create hardware RAID or software RAID?
According to this article the "WD Thunderbolt Duo 6TB" misses - in contrast to the Pegasus2 series - one essential thing to be able to create a hardware RAID: a hardware RAID controller(-chip).

So whether you use Disk Utility or the native Western Digital tools you will only get a software RAID. 

Even with an hardware RAID controller inside an arbitrary device, Disk Utility can't create/handle hardware RAIDs because it doesn't contain the necessary drivers to do so. AFAIK the only Apple tool for hardware RAIDs is the "RAID Admin" or "RAID Utility" accompanying the Mac Pro RAID Card and Xserve RAID Card.