PG is thin, lets through the most flavor, lets through the most of nicotine's TH and irritation, makes thin looking vapor
VG is thick, dulls/smooths everything PG lets through, makes thicker more persistent vapor
Most US juice companies offer at least one PG/VG blends and some offer every 10% blend
atomizer w/ cart or drip tip (drip tip for manual batteries only) - use any thickness as long as it wicks faster than you vape (if not then you must thin it)
standard stuffing cartomizers - do not like thick juices. I use 80pg/20vg or less VG. The hotter LR cartos that can only be used on fat battery models and mods (ecigs with 450mah+ batteries) may be able to handle slightly thicker juices since heat loosens juice.
reservoir cartomizers and tanks - 20-30% Vg are good levels for these. If a vendor thins their higher VG juices you may be able to use 50/50. 100% PG works but may leak more easily
With any juices - you may find that lighter clearer juices are less likely to leave flavoring sediement. Sediment will gunk up an atomizer faster causing it to need cleaning sooner. Sediment can clog the the carto stuffing causing sluggish drag and sluggish wicking sooner.