Depends on what your budget is and what size tanks you prefer! Beings as you are using dedicated tanks like I do, I recommend perhaps an assortment of bigger/smaller. I REALLY like Kanger Tanks for simplicity if you're not into rebuilding yet! It will be a DEFINITE improvement up from the Vivi Novi IMHO, as even with the bottom coil ones, I STILL experienced a lot of dry hits!
The new Kangers are bottom coil, and ALL a good choice!
Evod MT3-plastic version, so no citrus/cinnamon flavors on this one but EXCELLENT cheaper tank,
Evod 2-glass-upgraded version with glass/stainless steel, removable drip tip, uses dual coils, VERY NICE!,
Mini protank 2- glass, S.S, but more open look then Evod 2, removable drip tip, single coil, MAY have some probs with leaking, still a very good tank!
Protanks 2+, bigger version of the Mini 2, base can be upgraded with aeroflow adjuster which works VERY well!
Aerotank- my ALLTIME favorite of them! Bigger tank, sharp S.S and glass, aeroflow adjustor, removable drip tip, bottom dual coils, VERY low maintenance, NO leaks, rarely gurgles, and if so, it's been user error in my case, EASY to disassemble and clean, variety of replaceable colors, (mini/protanks also), performs and tastes VERY well for the price!
So there you have it, MY opinion and experiences of course! I haven't used the Unitank because it hasn't received as high of marks as the Aero, and I have no need to with the Aero's and my other tanks! I recommend looking up these tanks and deciding on try at least a couple to a few depending on your budget/needs, you really can't go wrong with any of them although I'd stay away from the 1st gen protank/mini! I can't WAIT for the newest Aero version, they look sick, and I'm SURE will perform EXCELLENT, and are a great price for the performance they'll have!
I buy mine off of Ebay, becareful though there's been a few people concerned about counterfeits, so ALWAYS buy from a reputable seller, with excellent FB and verfiy the security checks! ALL of mine have checked out. I buy the Evods/Evods 2 in a 5 pack for $15-20, and the Aerotanks for $17, sometimes with an extra colored glass tank even! I also buy my coils there for the Evod 2, aerotanks take dual coils, I get 25 for $33, and the Evods/minipro take single coils. I also get my coils at Discount Vapers as they just recently started carrying a large variety of Kanger accessories and replacement coils at the best price I've found and I've REALLY looked around!
Good luck, hope this helps!
-Feignix
Hey, I just noticed you are in rural Nebraska! From TX here, but am currently in "rural Neb" as well. Too cool, waving hello from behind a big haystack here! Soon, the view will be blocked by corn, and we'll be peeved every time we drive somewhere and get behind a big tractor!

LOL