The main difference between the two is the fact that NiMH batteries (the newer of the two technologies) offer higher energy densities than NiCad’s. In other words, pound for pound, NiMH delivers approximately twice the capacity of its NiCad counterpart. What this translates into is increased run-time from the battery with no additional bulk to weigh down your portable device. NiMH also offers another major advantage: NiCad batteries tend to suffer from what is called the "memory effect". NiMH batteries are less prone to develop this dreaded affliction and thus require less maintenance and care. NiMH batteries are also more environmentally friendly than their NiCad counterparts since they do not contain heavy metals (which present serious landfill problems).