I know some guys are using Lipos with a regulator these days, but LiFe's, IMO, are the more practical choice. They are a nominal 6.6v but in actual practice put out around 7.2v fully charged. A 6v NiCd puts out 7v fully charged, so they plane doesn't know the difference. You get all the advantages of a Lipo with no regulator and no fire hazard.