This morning I stumbled upon the North Woods Law TV show episode in which they found the body of Gerry Largay, who was solo hiking on the Appalachian Trail, stepped into the woods to "got to the bathroom", and couldn't find her way back to the trail.

Here is an article about her and her last days:
‘When you find my body’: The last days of Gerry Largay

I'm sure this story has been discussed here often. I find myself thinking about what she could/should have done different. I can think of several things involving careful use of a compass, ribbons for trail marking, ..., but in the end if she had invested in a Personal Locator Beacon, this tragedy could have been quickly resolved.

Isn't your life worth the $250 cost of a Personal Locator Beacon? Please, PLEASE, if you go anywhere that may be outside of 100% certain cell phone contact, buy a Personal Locator Beacon today!!

Well, a PLB certainly might have helped, but more fundamental to the issue was her lack of knowledge and expertise in the outdoors, to say nothing of her decision to push on solo when her companion left.

What would be a better investment - a PLB or a good WFA/WFR course or something comparable?

Why not do both?

the idea is to be self-sufficent, then be findable

survival kit
gps
PLB

Yep, I got my replacement PLB last week (1st one was in a backpack that was stolen from my car). My WFR expired several years ago although I still retain the knowledge.
The biggest resource you have is your own knowledge and awareness of your surroundings. From what I read about Gerry, she had problems with way-finding in the first place and was not suited to hiking by herself because of that. Technology and maps and PSKs would have helped but are secondary resources; knowing how to find your way is so key to keeping yourself found.

Yes,you can do both, but that wasn't the question. Which is more useful and applicable in a wider range of circumstances? Taking any course or even doing the research yourself and acquiring knowledge may not only require some money, but also time and a certain amount of dedication, which is a significant downside for some. Knowledge is very light and not likely to be misplaced or lost. Doesn't require batteries, either. That gets my vote.

What a crushing story to read. I'd read it before but forgotten some of the details. Mistakes were made to be sure; many of them trivial if made alone but as they stacked up it became tragic. The linked article was pretty good but still a bit incomplete, so maybe some of my assumptions are incorrect. But it seems she had no good topo map of the region nor any true understanding of land nav. She seems to have had no GPS, which combined with a bit of knowledge also could have made it a minor inconvenience instead of a years-long body recovery.

The media does indeed seem to consider anyone with their own backpack to be an "experienced hiker". Well, enthusiasm isn't the same as experience and love of the wilderness is not the same as knowledge. Knowledge is lighter than gear but just like gear, knowledge can fail you.

I don't presently own a PLB. I never venture [at the moment] to places far enough off the grid that I need one. But you can bet that if I get to take a more remote trip I'll be picking one up first.

As sad as this tale is, there's a small silver lining if it prevents someone else from making the same mistakes.

Garmin has a new Mini plb out...

Do you have a link?

I suspect you're referring to the Garmin inReach Mini

https://buy.garmin.com/en-US/US/p/592606

That is not really a Personal Locator Beacon, but rather a cousin of the Spot device. It uses the Iridium satellite network rather than the COSPAS-SARSAT satellites used by real PLBs.

If one is hiking alone and stops to go off the trail for bathroom business, why not tie a rope to a tree or bush by the trail and take it with you. That way it is easy to find back to the point where you came from. 20-30 meters of paracord does not take much room.