In reply to Milesy:
There will be a higher force using longer attachment method, however, if you are using dynamic rope, two cows tails out of some spare dynamic, then it shouldn't be a problem. This is assuming you have a decent anchor! Also, using two cows tails lets you never be unclipped from the loops.
I find that method a pain if the route is anywhere requiring both hands to stay on. The safest system i use requires two ropes. ( or one long rope doubled over with a figure eight in it to create to seperate strands). On one rope i use a Trango Cinch attached to the belay loop with a twistlock carabiner. And the other rope has a petzl ascension (or basic)with a 15cm sling larks footed to the belay loop and attached to the ascension with a screwgate through the two connecting holes. (Note, the croll is not suitable as it only has one hole at the top, not two.)
This method allows you to climb with out having to tie backup knots, or pull through slack (after the first few metres). Having the ascender on a 20cm sling allows it to never really be weighted unless in a failure of the cinch. This avoids excessive wear on your ropes. Also, protect the rope edges, wear a helmet, and carry another prusik and descender, just in case!
Hope most of that makes sense.