PG is thinner, makes quickly dissipating vapor, lets through the most of nicotine's harshness aka TH and the most flavor.
VG is thicker, makes fuller slower dissipating vapor, dulls the harshness of nicotine, dulls TH, dulls flavor
I like 70pg/30vg or for more TH 80pg/20vg, and I usually buy my juices from places that offer levels like that.
Re mixing PG and VG - yes - if you have a PG
juice and a VG
juice of the same nicotine level you can mix them - the flavors will blend if different flavors and the result will be the same nicotine level per ml. If you just add plain VG (USP Glycerin from a drugstore) to ejuice, then you will reduce the flavor and the nicotine level.
Nicotine is a skin irritant BTW. Getting any medium to high nicotine juice on your lips or anywhere else can make a burning sensation. And sometimes carts leak and juice gets on your lips. I prefer cartomizers. Once filled and settled they rarely leak. You still add juice when they need it to avoid singeing the stuffng and I blow out the air channel by puffing into the threaded end (else you can get a bit of juice on yoru lips and tongue right after filling). But in general, the juice stays more where it should. I use standard resistance normal stuffing-type soft-cap cartos (soft-cap so you can pop the mouth end cap and add juice). They are only reusable up to a point - you may get 4-7 days or more out of one carto.