There's no inherit advantage (other than people not thinking you're walking around with a dildo in your mouth, which is an advantage not to be underestimated), they're just build as a box instead of a cylinder. They tend to be more stable, if you have a heavy tank. Some have interesting features because there's more space; the Reos are neat, though also way overpriced. I wouldn't buy one, but they're neat. There's a difference between egos and the disposables you find at the gas station, and between VV and non-VV (but VV egos do the job fine), but I don't think there's much functional difference beyond that. I think it's like pipes and bongs, it's really mostly aesthetic but people feel strongly about their choices.
Thanks for elaborating. I have a VV eGo and two non VV eGos for backups and the only real difference I can see, apart from size, is that the VV eGo works better with the tanks I use (Kanger T2 and MT3S). Wow, that Reo Grand is as expensive as other (dildo) mods that I've seen. I guess I'll stick to the eGo (tampons?) I've been using.