JOYE 510 e-cigarette batteries: 3.1V
Tornado/eGo type batteries: 3.1 V
KR808D-1 batteries (and presumably most of the knock offs): 3.7V
Now, those numbers aren’t going to be precisely the voltage at any given time (a freshly charged KR808D-1 might come closer to 4.2V, for example) but you get the idea:
KR808D-1 effective voltage>JOYE 510/eGo & friends effective voltage***
So, if you like vaping at 3.1V, the eGo/Tornado might be your best bet… and, in fact, if you’re using a 510 atty, the eGo has another point in its favor, due to the basic design–but we do have another point or two to cover in this series, so I wouldn’t rush out to get one just yet.
If, all else being equal, you like vaping at an actual 3.7V, however, the KR808D-1 is a better choice.