Ok.. read about half of the info.... I am usually big with an explanation, but here goes....
Ejuice gunk can be caused from several factors. Neither PG or VG leave any residue to my knowlege. Nicotine doesn't leave much of a residue either. That leaves the flavoring as the major culprit for ejuice gunk.
I don't know how most flavors are made, but a lot of the tobacco flavors are made like a "tobacco tea". In essence, they take some tobacco leaves, put it in PG or VG VG, cook it for some time, then strain the leaf residue out of it leaving the tobacco flavoring in the PG or VG. As you can imagine, more than just the flavor can come out.. and when the PG, VG, and nicotine are vaporized, any extra residue will be left on your coils.
Sweeter flavors also may contain minute traces of sugar. Pure vanilla extract as an example has about .5 grams of sugar in 4 grams of extract. I am sure some goes up in vapor when you vape it, but the sugars that are left behind, is just heated up on the coils.. which caramelize. Of course there is Imitation vanilla extract, but that too can be made from other organic substances, but probably contain much less sugars than a true vanilla extract.
so, in short, the flavoring has a lot to do with what is left behind. If you vape certain flavors, you're probably going to get more of a build up than others. My main flavor is Guava. The juice is clear, and I go about 2 weeks before even looking at the coil. I use 28 gauge wire as my main, and clean my coils about once every 2 weeks.. and even then, they don't NEED to be cleaned, but I do it anyways. The cotton I pull out is mostly white, with it being a light tan color under the coils. When I vaped RY4, the wicks would be dark brown, the coils black, and this was after one week. Back then, I used Silica, so I would torch the silica and dry burn the coil. I would replace the coil every 2-3 weeks.
When choosing a flavor, I always buy a small test flavor. The darker juices usually have more particles in them than clear ones, and are probably more likely to gunk up your coils. There is no surefire way to tell. If you want to give it a try... For a day, try just vaping pure VG on a fresh coil. It will have a slightly sweet taste. If you have a second Ecig, use that to get your nicotine if you need it. See how long the coil lasts in just one day.