What you should be concerned with is the ability to provide the necessary current under load. A 1500 ma NiCad can provide more current to the digital servos than a 2700 Sanyo Nihm and on-par with a single 2400 Fromeco li-ion. A new low internal resistance 1950 Sanyo Nihm is smaller and lighter and will out perform both the 2700 Nihm and a single 2400 Li-ion Fromeco. (most people double up the 2400 Fromeco to get 8amps of current needed for mutable (6-7) digital servos). Lipo's on the other hand have very good ability to provide high levels of current, but have not yet been accepted buy the large scale RC world. For 3-5 digital servos you are OK with about any of the choices mentioned.