For all the reasons Kiwi has mentioned, I tend to agree that the Darwin, while great in it's infancy, has been overshadowed by modern VW devices. The menu system was good, but the design was awkward, and the arm? Really? That thing is ridiculous... It's very cost prohibitive, if you can even find one... No replaceable battery just sends it on the downslope of the hill, IMO... So with current day options, I wouldn't even consider the Darwin...
There are relatively few "mainstream" VW devices out there right now... The ZMax not-so twins... Anyvape CVI, eVic, VAMO. Probably some others I have failed to mention here...
Now, this is just my opinion, but I prefer the Vamo for ease of use. The UI of the SmokTech ZMax is similar to the ProVari, and the Sigelei and Anyvape UI's are almost identical, all three of which use a single button to access all features of the APV. It takes a lot of button presses to get what you want. It may be "easy" to perfect the UI, but pressing a button 4 times to get to option "X" is not as easy as pressing a button once to perform the exact same option. In fact, having to press one 4 times is quantifiably 4 times more difficult than it is to press it once... I'm sure you get the point... LoL... The eVic has a pretty revolutionary UI, and really it's more of an OS in there, with software upgrades and the whole bit... But, it's far from perfected, and IMO Joyetech still has a lot of work to do on the eVic UI, in addition to the hard electrical limitations of 5V / 11W max settings (most VW devices have 6V/15W max).
The Vamo has three buttons... A Fire button, and a plus and minus... Anything you want to accomplish is easily done within seconds... You pass up the number by .5W or .1V? No problem, go back the other way... With the single button you have to back out of the menu system, go back to power up or power down respectively and press away again...
They all have VERY similar performance, eerily similar in fact... All operating their PWM circuits at the exact same frequency, one could assume they all use the same circuitry to control power regulation. So, with that assumption in place, you can assume that accuracy and their ability to fire your attachment of choice are all going to be the same. Really only a few other factors to consider after that... Build quality, in which case I would say the Sigelei and SmokTech ZMax are probably the kings. Appearance... Again, I like the ZMax... Ease of use, I like the Vamo.
Ultimately when I made my choice, I weighed ease of use higher than the other factors. The Vamo just also happens to be the cheapest of the lot, so that was an added bonus.
Now, all that said, if Joyetech ever fixes all the things I don't like about the eVic, being the big geek I am, I'm gonna have to jump all over that...
Really, it's so subjective with these things... In the end they all apply current to a tiny coil to vaporize liquid, and in a blind test of vapor quality from one to the next, I'd contend you will notice no difference at all from one to the next...