I used to spend $1500-1700/wk shipping wholesale auto parts in the southeast US. You get in a lot of hot water when you tell a customer they'll have their part by x day and they don't get it--because now they have to tell their customer why their car isn't back together and another customer why they can't work on a job they already promised to start on because the bay isn't empty. It snowballs quickly.
We originally started with Fedex ground, and they were HORRIBLE, taking up to 9 days to ship a package not even 1hr away by car. After being burned here, we tried DHL ground. They were better, but not by much. Both of these carriers 'last mile' deliveries are not by company owned personnel, but contractors. So quality is very, very hit or miss. Neither of these companies had a guarantee on their service.
Then we tried UPS. Not only did UPS let us know to the penny how much a shipment would cost, they also guaranteed that it would get there on the date they said it would or our money back. Over a period of 16 months and hundreds of shipped packages, we only had to get a refund for 2 packages due to late arrival, and only 6 due to damage in shipment (granted, we did pack things very well). Overall experience? I always, always prefer UPS. Sometimes waiting a month for something is okay since the timing isn't that important. But when it matters--UPS.
When it comes to overnight deliveries, Fedex and UPS are both super options with Fedex usually being cheaper. Fedex's overnight services are what they started with and it's why people say 'fedex' something like 'xerox' something--they built the idea of overnight services. UPS has done well with competing with Fedex in this area, but Fedex is still what we find to be the best value (for documents). For packages, UPS is better and offers more services like 3-day, 2-day, and three different levels of overnight including the crazy someone-will-jump-on-a-plane-with-your-package-to-get-it-to-right-now option.
DHL is still the carrier of choice for International as that was their bread and butter for decades before they bought out Airborne express and RPS (roadway package services). I haven't had to use them yet for International, but do have a nice terrible experience with USPS on that.
USPS to me is a big cheater. They compete with PRIVATE companies using government money. I have no problem with that if you want to let UPS, Fedex, DHL, and others start to deliver mail. I'd love to pay UPS 15 cents a letter and have it arrive two days faster because I know that's what would probably happen. The 'smartpost' services are essentially a hybrid service that uses the private logistics lines that are faster/cheaper than USPS in-house operations. USPS still has the best 'last-mile' reach as they do service every address in the US daily (which no one else does). But the way they've cut into the revenue stream of UPS and Fedex rubs me the wrong way since I know how hard these companies work to earn the customers vs the USPS government attitude towards their customer base.
And USPS tries to compete with Fedex and others on the overnight business, but fall ridiculously short. It would have been $45 to sent a document overnight to India via Fedex. My wife insisted that we save some money and use the post office. The document was supposed to arrive in India in 10 days. It never did. Two months later when we were physically there ourselves,
I signed for the package myself in India. It was a joke on the highest level. USPS overnight/express services are a joke, especially International. Their registered and certified services aren't bad, but there's no real recourse if something goes wrong (like anything with USPS).
Sorry for the super long sharing of shipping experiences, but I hope you are better equipped to choose the right carrier for what you need.