A lot of posts have said the lava tube is a good "stepping stone" into VV. I assume this is due to the $70 price point instead of $180 for the provari or $245 for the darwin.
What do these other 2 have that the lava tube doesn't? The battery itself is purchased separately, so all battery life/dependability is out of the question. All the protection specs look similar. What else? All they really are is a tube with a variable boost circuit, no? As long as the device accurately creates the voltage you dial in, that's pretty much a wrap.
Perhaps long term reliability (including quality of parts and craftsmanship) is the difference. Still, at a price point of less than half, it would have to be a significant difference.
FYI: I have never tried any of these. I'm looking into my first VV also and these are some of the questions I've been coming up with.
The Darwin is a little different (variable watts instead of volts), but overall you are correct that the difference is mainly in quality of parts and durability.
You left out some other VV devices though: boxmods (that I believe can be even cheaper then the Lavatube) and the NotCigs products that will cost $120-$150 for a full kit (with batts and charger).
It's just a matter of finding the style and build quality you're looking for.