I'm so glad you posted this, as I was struggling with the same decision. I already had/have the Joye 510 and was considering the Joye eGo when someone mentioned the hello 16. When they mentioned it they sent me a link to a site where you could just buy the battery kit for about $35 so you get the battery an atty and a charger. So I figured I'd start with an eGo kit which I got and I LOVE (just got today) and even though the battery life is more than sufficient for my needs right now (I think, like I said I just got it today) I plan on buying that battery kit when I have some extra cash and checking it out just for the heck of it. I don't mind it looking like a mod, so I don't really care about the cone, so I figure it'll just be something I can use when I'm out and about with no power source for a long period of time.
Just FYI, in the eGo manual it says that your battery should last anywhere between 5-10 hours. I started vaping mine straight out of the box and was pretty much using it as a pacifier (I can't help it, I love vaping) and it did last about 5 hours. And no joke when I say I literally didn't put it down from the time I took it out of the box till I plugged it in. And in my experience with batteries (not neccessarily with pv batts cause I'm new to all this, but with batteries in general, especially cell phones and other lithium ion batts), when they come precharged, they only have a partial charge on them. So if that's the eGo's batt life on a partial charge, I'm very impressed.
Good luck in your decision making, and I'm sure you'll be happy either way you go