Flavor, vapor production, and allergies. VG tends to produce considerably more vapor, but mutes flavors and throat hit. It's also very thick, which makes it feed and wick more slowly, which can result in dry/burnt hits if you are hitting it fast on certain atomizers. PG on the other hand is very thin, feeds very well especially in tanks, and carries flavors better, but does not produce nearly as much vapor. Some people get throat or respiratory irritation with too much or any of one or the other as well.
I wasn't trying to bring you down or be negative, but people have been chasing 0mg throat hit solutions for YEARS and the best anybody has come up with is imitating something sort of similar to it. Unfortunately, that just happens to be equally true in the opposite direction.
Edit, to respond to your first question: Full 100% vg juices tend to taste sweeter, but have less flavor if that makes any sense. It has it's own natural sweetness, it just doesn't carry the flavors that are added to it as well as PG. They do produce really pretty big puffs of vapor though. They also can be very slow traveling down a wick, so you end up having to VAPE, wait... VAPE, wait...