If you order juices with a PG/VG mix the amount of PG will help determine how thin they are.
PG is thinner and contributes to throat hit (TH.) It carries flavor a little better. Some people find it too harsh, especially in higher percentages.
VG is thicker and helps to add a bit of sweetness and smoothness. It also increases vapor production, but it can mute some flavors because of its thickness.
I use either a 50/50 mix or a 60PG/40VG mix. I'm not a big fan of TH and I've found I'm a little sensitive to mixes with higher amounts of PG in them. I don't have any trouble filling cartos with either mix, but it does take a little longer for the 50/50 mixes to soak into the filler. Not a big deal for me because I'm patient and subsequent refills go a little faster because the material is moist.