The Peapod style is lighter than an over and underquilt. They can both be used as a warm coat round camp if a head hole with velcro is provided, reducing the need for a warm jacket.
The underquilt takes more tweaking to rig it properly for very cold weather, so that there is absolutely no air gap between hammock and underquilt.
Both are great ideas, but the peapod can only work on open hammocks like the eco system or the group buy hammocks. Hennessy hammock users have to use underquilts.
Both can be used on the ground if need be, assuming you carry a bit of foam or sleep on your pack. But if it gets too cold for the hammock, both the over and underquilts can be put on top of you on the ground, to provide maximum insulation, while the peapod would have to be used as a regular sleeing bag.
Since I already have an underquilt I wouldn't mind a peapod
but that's just selfishness. I'll post some pictures of my underquilt setup tomorrow when there's some light for the camera.
In the meantime:
The JRB nest down underquilt, which attaches to the Hennessy entry:
http://www.jacksrbetter.com/index_files/Products List_files/DSC00382 The Nest Slit 2.JPG
The kickassquilt, synthetic:
http://www.kickassquilts.com/images/potomacFull/underside2.jpg
The Speer peapod (scroll down to see pics):
http://www.speerhammocks.com/Product Pictures/product Pictures.htm