don't forget henges don't always have stone settings within them, and some stone circles aren't in henges.
but i'll go along with steve, to some extent that major earthworks are pushing the limit of technology...perhaps. because if they were being constantly improved why stop at just one bank and ditch? (although i suppose you could argue that building a 2nd or 3rd like at thornborough is the improvement, or the setting of stone circles within the henge).
what fascinates me is the idea that it is the ditch that is the important aspect, as opposed to the bank. lots has been written about the effect of the bank generating a false horizon, or open 'church', but some believe it is the ditch that was more important. brodgar has a ditch cut into the bedrock, and if they are descended from causewayed enclosures perhaps the ditch is of significance more than the its spoil heaped up in a bank.