I'm going to suggest to you that you buy pass both of these devices and get yourself two eGo USB passthroughs. These arent real passthroughs, instead they are simply 650 mah batteries that you can use while charging. They go for about $22 at my vapor store. They charge via usb and are 510 threaded. I've had two of these for going on 7 months now, not one single issues with either.
I had an e-power but like another poster said, it's collecting dust. The button on the e-power to me is aggravating. It's like almost flush with the body, so say you are driving and night and really cant look down to find it, well you can't feel it either so its a pain in the ..... The genuine eGo has a raised button that you can feel for and find easily without your eyes. The eGos I own are also smaller about the size of a lipstick tube. Whereas the e-power is bulky. I've also had some of the cartomizer/atomizer issues mentioned above. It just behaves wonky.