Lead acid is most definitely NOT high capacity. NiMH, or NiCd have higher energy densities. The reason lead acid is desirable is because it's really robust (Think about the one in your car and look at the conditions it operates in), and relatively inexpensive when compared to the other two. Even though the energy density is low, it's usually more cost effective to just give up some extra space in the house rather than paying a lot more for a smaller pack.

Also, could you show me where you got the 6 month environmental cost for solar cells? (I'm not trying to be smug or anything, I honestly been looking for this kind of information for a long time).