I would agree the first sentence here, but quibble with the second.
I was stunned to see that anyone felt that the USPS offered decent service, double stunned to see it listed first by most people. Not disputing anyone else's experiences, but it's certainly not like that here. I reside in one of the nation's highest income counties, and USPS service here is not just third world-ish, it is hellish, so the idea that poor neighborhoods get picked on might be due to who staffs them, not bias.
In the last 25 years, at least 90% of the packages I have received which were handled by Postal service employees have been crushed. No exaggeration, and closer to 100% than 90%.
No "Priority Mail" package into here has ever reached me in less than a week, and that speed is rare. Ten days is the norm, and three weeks is fairly common. You will never wait in line less than 20 minutes, 45 minutes for the month of December.
On the other hand, and I ship over a hundred boxes a year of various weights and sizes, from 1 pound to 50+ pounds, via Fedex, and in addition to being hyper convenient locally, I have never had a single instance of the slightest problem. That is literally thousands of shipments. Plus, if I use FedEx Ground, and take it to the actual FedEx office, it is significantly cheaper than Priority Mail, gets there faster, and it has actual along the route tracking which USPS hasnt figured out yet.
So, a lot probably depends on where you are. I'd imagine those extolling USPS and disparaging FedEx would say I was just lucky, and I'd be forced to say just the opposite.
Use what works best for you where you live, but blanket recommendations are not realistic.