The way I look at is that there are 4 types of resizing:
Partial Neck Sizing - sizing only part of the neck which is what you can do with a FL die backed up 3/4 turn off the shell holder. That will size about 3/4 of the neck and not size the case body or hit the shoulder. This can only be done until your case develops a crush fit at the shoulder unless you don't mind forcing the case into the chamber with excessive bolt force after the case develops a crush fit.
Neck Sizing - sizing all of the neck and this can only be done with a dedicated necks sizing die. This can only be done until your case develops a crush fit at the shoulder unless you don't mind forcing the case into the chamber with excessive bolt force after the case develops a crush fit.
Partial Full Length Resizing - sizing all of the neck, sizing the case body and pushing the shoulder back a minimal amount leaving some contact at the shoulder between the case shoulder and chamber shoulder for a very slight crush fit.
Full Length Resizing - sizing all of the neck, sizing the case body and pushing the shoulder back beyond all contact.
Personally I PFLR by pushing the shoulder back ~.001" which gives a very slight crush fit but will hold the case in place by being trapped between contact at the shoulder and case head against the bolt face. That's where I have found my best accuracy but it will not guarantee accuracy unless you take care of all the other factors such as run out, bullet grip, annealing necks, powder choice, primer choice and bullet choice.
"What criteria do you use for full length vs. neck only resizing and why?"
Two criteria: One, which one gives me the most concentric ammo. Two, how easily it chambers for the use I will put it to.
Many "expert" shooters/reloaders have 'proven" that FL sized ammo is fully as accurate as neck sized. I wonder if they didn't just prove their neck dies were no better than their FL dies. The ONLY neck sizers I've found to make straighter ammo is the Lee Collet Neck sizer, and it has to have some good cases/necks before it can do much either. No die can make concentric ammo using cases with non-concentric necks.
Meaning, no method is an automatic improvement. Do it, measure the ammo and shoot some targets. Otherwise you may just be kidding yourself about what you are accomplishing.