B6's used to be classified as Heavy Duty or Sport.
Heavy Duty's stayed as B6
Sports became B8
The high pressure on the monotubes means you may feel more imperfections of the road compared to twin-tubes.
Depending on who you talk to, some at Bilstein say B8's, while having a shorter shaft for lowering springs, are also valved for a more aggressive compression and rebound profile than B6 HD.