I now have both, so I can speak from some experience. My 2 cents:
VK Pro's:
Battery life
cartridge life
ease of use (assuming you discard carts)
MUCH better flavor
"Warmer" feel to vapor
Cons:
requires long, slow drags, sometimes you need to "double clutch" the battery
While cartridges can be refilled, the fill tends to get burnt if you let them dry. Reported life ranges from 3 refills to 10 plus.
510 pros:
More vapor, at least on a fresh battery and cartridge fill
experience more like an analog, you can take much quicker hits
you can drip!
Cons:
higher learning curve, messier, more upkeep (unless you are refilling carts on the VK, then it evens up some)
Abysmal battery life
less cartridge capacity (can be offset a bit by doing
mods)
In short, If I had started with a VK before my 510, I would have never looked back. I have sent at least 5 people to V4L so far, perfect for people new to
vaping. BUT, there IS something about the 510 that keeps me coming back. My "ban" stock now contains supplies for both.