I think the more applicable comparison here is between the E-Nano and the Underdog because they have more similar heatport setups and similar milking abilities. The heat island has a very small thermal mass that causes it be far more difficult to use effectively for big hits, at least in my experience.
The real difference though comes down to AC vs DC. The Underdog and most other vapes are running from DC power sources, while the epicvape line exclusively uses AC power sources. DC powered vapes are more flexible in terms of the power sources they can support - car power, various batteries and solar are all options for powering a DC log vape. That being said, the AC powered vaporizers are generally cheaper to manufacturer.
I should also mention that these "DC" vaporizers are totally capable of running off of AC power sources; but only low voltages - not the line power that the eNano is made for.
As well the dimmer on the eNano is not really the same thing as the VVPS's. A dimmer switch is just a linear resistor on a knob, so your power will not be exact; things like line fluctuation and temperature will cause the output power to drift a bit. The miniVVPS and Voltage Keeper (and related units) have a small computer in them that poll the input voltage several times a second and actively compensate for any drift it sees for a stable output source.
Other than that, the only difference is the ohmage of the heater (still runs around the same wattage though) and the ability to get custom woods.
I should mention that I am biased towards the DC powered logs both from a business perspective (I make power supplies for them); and from an electrical engineering perspective due to their inherent safety.
