Hi Andrew,
LiPo has an energy density of ~ 250 Wh/L while lead-acid has 80 Wh/L
http://www.altenergymag.com/emagazin...lications/1884
This figure is in litres, so weight for weight, the differential is even higher between LiPo and lead-acid.
The big question for me is safety - LiPo batteries must be carefully controlled for charge current and temperature or they can get very hot and self-immolate. And they must not be discharged below a certain point, which means the battery controller must be quite a bit smarter than an equivalent lead acid controller.
Regards,
Tony Barry