Just the battery life. However, it also depends on whether you are talking about eGo or a
mod. eGo, Riva, and Hello 016 are all, as far as I know, 3.4 or 3.2 volt. Mods are usually 3.7v, unless you deliberately go for a 5v or 6v
mod, which uses more than a single battery. That does not sound like much difference, .3 or .5v, but most find it does increase vapor production. (But may diminish flavor slightly, and be more prone to a burnt taste, depending on how careful you are to keep the atty or carto wet.)
All batts also have some voltage drop near the end of the stored energy, and vapor drops off.