I've done both private practice and ED scribing.
Personally, I quit the private practice and went back to ED for a few reasons:
1. Private practice is CRAMMED for time. Seriously, patient visits are in-out-what-was-your-name-again? Maybe it was just the doc I had, or maybe the specialty, but I found it very frustrating. More to the point, the doc had little down time between patients - meaning no time for you to pick their brains. I tried switching because I thought it would be more chill - hey, the hours were shorter, the schedule regular, what's not to love? It. Was. Hell. The day just felt so busy and meaningless.
2. Private practice exposes you to ONE doc, maybe a few if it's a group practice. If you scribe in an ED, you will likely get to know several ER physicians very well. This is particularly useful when you're talking LORs...much easier to find one doc out of a group who you really click with than to hope you get lucky with the private practice physiciah.
3. Shadowing opportunities in the ED: You call the consults, so you will get to know ALL of the hospitalists, plus at least 1 or 2 from each of the following: general surgeons, ENT, Nephrology, Heme/Onc, Cardiology, Neurology.
4. ED, by its very nature, exposes you to the broad ideas of a lot of different things. Private practices tend to be a lot more narrow and detailed. Now, since you're talking FP, that may not be the case...but keep in mind that the ED IS primary care for a lot of patients, so it's kind of the best of both worlds for scribes.