For all portable vaporizers that can fit inside your pocket, it would seem that smell is the most important consideration in the "stealth category".
Since FF2 gives off the least smell (convection rules!), it would stand to reason that FF2 is the most stealthy device on the market.
People say the Pax is the most discreet but have you ever kept one of those in your glove compartment of your car? You get pulled over and you're busted. The Firefly on the other hand does not give off the scent of cannabis.
In short: The Firefly was meticulously designed, and with this comes airtight seals and an odor proof device.
I would agree with the other answers on smell. The firefly does not smell at all. Which is great the stealth.
My best trick for making the firefly look more stealthy when out and about, is the way you hold your firefly. Hold the firefly with your hand over the top of the firefly gripping it on either side. That way if someone sees you they wont be able to clearly see what your holding. I used to do this with the FF1 and it worked a treat. As the FF2 is even smaller it should work even better !
All the compact personal vaporizers on the market are similar in terms of staying below the radar. They look extremely similar to all the other "vape products" on the market and to someone with an untrained eye, they likely would not know the difference.
Since the other answers are directed toward the smell - I'd say a lot of this has to do with how well you keep your device clean. The Firefly DOES seem to go a step further than the competition through their unique design of the device, which seals the chamber better -- but if you have a dirty vape there will always be some faint smells escaping. Also, take into consideration the oils that get deposited on the EXTERIOR of the device by the user's fingers/hand, these volatilize the odor of cannabis as well.