I own several versions of the vision vivi (slim, large, plastic, plastic with metal sleeve, metal tank, etc, etc) and have 2 protanks. I love the ProTank atheistically and with the EVOD head the draw is about perfect for me, no dry hits but the metal tip is a turn off for me. I get a metallic taste no matter what with the protank....its not a deal breaker, but the major con for me as I have never liked the metal drip tips be it stainless, aluminum or whatever. It does produce huge clouds of vapor though, YMMV.
Vivis on the other hand I do have some dry hits but for the most part they are good, nice warm vapor and good TH/flavor. The vivi wins for me for ease of cleaning/rebuilding/replacing, it's essentially modular meaning every part can be bought and replaced individually (heads, tanks, base, top caps, drip tip). Did I mention easy to clean/swap flavors?

The standard version also holds more liquid than the protank.
I'm also afraid to leave the house with the ProTank fearing I'll break it. Havn't broke a vivi yet, and even if I did no biggie I have spare tanks

Sorry this post is all over the place, kind of in a rush to post this as I'm headed out to work as we speak.
Cheers, and as always, Happy Vaping!