I've actually seen ones without the battery that are 3.7V also, so I wouldn't assume it's 5V unless it says so. I'd go with the 5V or the ego, benefit of the ego being you can detach the cord and use it like a reg ego battery. Also, from what I've heard if you throw a LR atty on it you get the feel of a 5V. I haven't done this, though, so I can't say that for sure. I also agree the reg 510 is kind of meh. Btw, the sit and go has been out of stock at madvapes for awhile, so you may not have that option at the moment.