how often do u shoot? that's prob going to give u the answer...
sanyo enelopes last longer coz of their ability to hold charge well... powerex has more power, but if u don't use it often it'll lose it's charge...
so if u're the kind who won't use it for a long time and don't want to keep having to remember to charge ur batts.. i'd suggest the sanyos...
personally, i use the powerex but i use it for my flash so the extra power comes in handy...