In reply to UKC Articles: Thanks for all the comments, its good to see people reflecting on the implications. Basically there are several types of prusik "knot". Each have pros and cons. And cavers have different rigs for rope climbing, which are more effective for SRT but tend to take longer to set up and work better on free-hanging terrain. The classic prusik is a good all-rounder but has a tendency to lock under load and so can be a bit harder to slide, also there are times when it is useful to be able to cup your hand or use the slot of a belay plate around a prusik to allow a controlled slip. The classic prusik is not particularly good for this. The "french" prusik or "autobloc" is great for this, but for uses where a slip due to the knot touching an edge would be bad news, this prusik could be very bad news. The Bachmann, which incorporates a karabiner, is a very nice prusik, having many of the advantages of other systems, but it is slightly fiddly to tie. Finally, the Klemheist and Kreuzklem are similar to each other and are made in the same way as a french prusik but finished by threading one loop through the other, so only the one loop is clipped. These have several advantages: they can be slid under load but less easily than the french prusik, and can be very effective when tied using a tape sling.
As a general rule of thumb, I tend to use an easy slip prusik for my leg loop (usually a french prusik or a klemheist) and a more reliably locking prusik for the waist (usually a klemheist)However, the Bachmann can be good for both, and the karabiner makes a handy grab point for sliging the prusik up. There are no rights or wrongs here, all the prusiks work but I would caution against using 2 french prusiks - the length of the prusik loop is fairly critical, if too short it tends to lock too much and if too long can slip unexpectedly - when 2 are used, if the top one slips, it will hit the bottom one and definitely will slip as well! Hope this helps.