Harald's recent post http://www.realskiers.com/pmtsforum/viewtopic.php?t=441 explains the mechanics of these two turns in an excellent way.
In my own skiing, I've noticed that WR for me requires a "pure carved" turn, that is, no "brushing" or skidding present. As soon as there is a brushing component in my turn, I don't feel enough in control or balance to execute WR. So, for instance on icy steeps, where I'm unable to achieve pure carved turns, the SP will be the release of choise.
I can only speculate re. the reason not being able to execute WR in coujunction with brushing, maybe the WR demands a very stable platform (non slipping) for the stance ski ?
Cheers,
T