You are correct. No vaporizer will put out smoke or the typical pot smell unless the temperature gets high enough to light the herb. That shouldn't be a problem except accidentally, once in a while. And even then, the amount of smoke will be very small, far less than what a joint puts out. Vapor does have a smell, but it's very light and disipates quickly. Someone sitting next to you while you vaporized would smell it, but someone entering the room 15 seconds after you exhaled probably wouldn't.

I can't comment on any of those vaporizers you mention because I haven't used them. However, while deciding, don't forget to consider how much you use. A vaporizer that might be just the trick when you a couple of stealth hits may turn into a royal pain to use throughout the day, every day, or it may just not stand up to heavy use.

Regarding efficiency, I tend to think they're all just as efficient. After all, they all get hot enough to go from the vaporizing range to the combustion range. There's a vaporizer out there that claims it takes the vaporized remains from well-known vaporizers like Volcano, etc., and manages to squeeze another couple of bags from them. I really doubt it.