1) What benefit does a double coil cartomizer provide as opposed to a single coil? Does juice type make a difference as to the type of cartomizer one should use? My juice preferences lean toward a sweet/fruity vape the vast majority of the time.
* The advantage is that it can give you more TH and vapor. Although, unless your using a DCC at 5V then it's really not going to be performing to good. Since there are two 3 ohm coils being used to make the 1.5 ohms. If your thinking about getting a 2.4 ohm DCC then they are better at 6V. With 3.7V then it's a sub-par vape.
Another thing to remember is if you plan on using this on an Ego type batt then you risk killing it since the switch is only 2 amps. Anything that pulls more power then that can damage it eventually. For example, a 1.5 ohm atty/carto at 3.7V = 2.3 amps.
Also, it burns a whole lot faster then a single coiled carto. Overall, the advantages of a DCC is immensely overwhelmed by it's disadvantages.
* With all cartomizers. It's best to avoid high VG juices since that will clog up the filler material and cause wicking problems. Dark and sweet juices will leave sediment in the filler material that can also contribute to it being clogged. These lead to a decrease in it's lifespan and also, the speed in which it can become burnt.
You can definitely use all of the ones that I have mentioned above in your cartomizer. Just have to remember the problems that come with it. Also, if your planning on using a carto tank. Then I would suggest punching two holes into your carto or making the slot a bit bigger to allow better juice flow. This can help with wicking issues. Also, let it rest more often so that the juice can seep throughout the filler material.
2) I'm going to be buying an ego/riva style unit. What kind of cartomizer would best suit my needs?
Boge LR 2 ohm cartos would be good or you can get the Ego Mega 2 ohm single coiled carto. Which is basically a super sized Boge carto:
Electronic Cigarette - Cartomizers - Smok Tech Cartomizers
It'll be the second one from the last on the page. They work great and utilize the best out of the 3.4V (ego) and 3.7V (Riva) that your gonna be using it on.