I'll give it a shot. These are my experiences and your mileage may vary.
TOP COIL
SplitFire
The SplitFire is a 3ml top coil clearomizer
tank with replaceable heads and double chambers. The top coil is in the top chamber while the
juice is in the bottom chamber. The two chambers are separated by a metal, double threaded "washer", if you will. This "washer" has four holes through which the four wick strands coming from the coil go through to reach the
juice in the bottom chamber. Here's a picture:
http://www.gotvapes.com/images/splitfire_1.png
Pros: Fantastic taste and vapor, no leaks. Really cool looking and easy to disassemble, refill and clean. This is my favorite as far as vapor and taste but see the cons below.
Cons: Replacing the head means you have to thread the four wicks through the holes. Replacement heads come with plastic caps at the wick ends, like shoe laces, and are easy to insert. But if these plastic caps fall off, it can be difficult to insert the wicks through the holes. Since you have two tubes, upper and lower, sometimes you can unscrew the other tube instead, not a big problem but it can take a little practice to get it right. When refilling, you unscrew the middle metal "washer" (divider may be a better word) from the bottom tube and thus you pull the four wicks out of the bottom tube. The wicks will be wet with juice and can be a bit messy. The plastic tubes although strong are not the strongest plastic (I forget the name of the plastic) and can crack under some juices.
Vivi Nova
This is a 3.5ml top coil clearomizer tank with replaceable heads in a single chamber. It has threaded top and bottom caps that can be unscrewed from the tube. To refill you unscrew the top cap and just refill with dropper or anything. I've just poured juice into it from a bottle. Screw the top cap back on and you're good to go. To clean and dry burn or to replace the head, unscrew the tube from the bottom cap to expose the vent tube with the head on top. The head can be unscrewed from the vent tube and a new one screwed on. The mini version of the Vivi Nova is not as easy to fill but otherwise performs very much like its bigger version as it uses the same heads.
Pros: Excellent taste and vapor, no leaks. The taste may be a bit muted compared to the SplitFire but not bad at all. I use this regularly because of its ease of use. Easy to refill, clean, dry burn and replace heads.
Cons: The plastic tubes although strong are not the strongest plastic (I forget the name of the plastic) and can crack under some juices.
Smoktech Titan
Very large, about 5ml I think, top coil clearomizer tank very similar to the Vivi Nova design, no leaks. Comes apart like the Vivi Nova.
Pros: Excellent taste and vapor, as easy as the Vivi Nova to refill, clean, dry burn and replace heads. It comes with the sturdier plastic (smokey in appearance, not transparent like the Vivi Nova) and can withstand stronger juices without cracking. The threads on the tube are metal not plastic. I use this regularly although I haven't tried it yet with VV juices since I am tasting new juices and the 5ml capacity means commitment to the juice. I will try Kona Coffee Milkshake on it soon. I expect an excellent vape with VV juices. Here is a picture:
http://www.gotvapes.com/images/titansmok_1.png
Cons: Because of its large capacity and the type of juice, juice may become slightly burned and change color towards the last half or third of the tank.
BOTTOM COIL
The bottom coils I have tried are the T3, Vapeonly, Smoktech Shorty, Phoenix Bottom Coil, and the Nano. The EVOD is coming. I would recommend the T3 and the Nano. From everything I have read the EVOD will be a winner too. I will stop here since this has become quite a long post and I don't think bottom coils are completely hassle free like the Titan, and (mini) Vivi Novas (just my opinion, many love them to death), except the Nano which is pretty much hassle free once you go through a tankful or so to clear the primer, but the T3, Vapeonly, Phoenix, Nano, and EVOD can all be excellent. The only one I would not recommend is the Shorty because the wicking material can burn and you end up inhaling that (it has some kind of mesh like a carto, not silica wicks like all the others). The Phoenix requires a syringe to refill and can be a hassle (for me it is).