I found a comparison on-line that had the tech specs for both the 680 and the 681 which showed the 681 being slightly heavier and slightly longer when collapsed although it does offer 161cm when extended as opposed to the 680 which offers 154.
I decided (with tape measure in hand) that as I am on the not so tall side that the 680 would do and was smaller and lighter when compacted.
Ah well, I'll see tomorrow when\if it arrives but I'm sure either the 681 or the 680 would do a great job.
One thing that has bugged me....
I got a quick release head for my Benbo and was pleased to see you could get get extra quick release plates. I expected these to be the base plate (single camera, multiple supports) only to find that they are actually the camera plate for those that have multiple cameras. I would have thought I could have got an extra base plate, one for tripod, one for monopod.