Hi Novio, I have a pro-tank which I like, also used mainly on a vamo - it looks good on there and gives a nice airy flavorful draw. I also love my vivi nova and my EVOD is pretty good too. It all depends what you are looking for. (I also like CE5's that a lot of people on these forums seem to not prefer)
If you like the cool airy draw on the pro-tank, you'll like the EVOD, it has a similar draw and the added bonus that any replacement heads that you buy for it, will also fit the pro-tank too. I've had no leaking issues, and it's easy to fill.
The vivi nova is a top coil device and personally I like the hotter vape it gives me for some juices (extreme ice is permanently in my metal mini nova and it packs a punch)
Like you said, it's subjective, but If I were you, I would try a different delivery system. People can describe how things taste and vape, but until you try them, you'll just not be able to make a decision which you prefer. You should be able to get a vivi nova and another bottom coil device like an EVOD or a T3 for about the same price as a pro-tank.
Finally, if you are going to buy a vivi nova, make sure you are buying the genuine version (my vapor store is where I get mine but there are other slightly cheaper place that I cannot vouch for as I've never used them) Some people have had leaking issues with the copies.
The other option would be to just spend the money you would have spent on all these new delivery devices and just buy 5 or 6 new heads for the pro-tank - they are usually 2 or 3 bucks each.