The HD is a standard clutch with heavier clamp laod,
The SO comes with an uprated disc as well as the heavier clasmp load for approx $15 more.
It's a better clutch in that it last longer & can take more abuse.
(No brainer, always use an SO over a HD)

As for price,
I sell them for $460 + freight. (Or $906 - fitted)
Or if you find a better price, take it.
Part number you're after is - HCK 7111 SO