I recently went through this with a trade here on TB. The guy I'm trading with shipped via UPS, was charged nearly twice as much as I was, and it'll get to me 3 days later than mine will get to him! I shipped via FedEx, btw.
Also, I've had only one "bad" experience with FedEx... they had my shipment showing delivered on the tracking, yet I had nothing! As it went, the guy came about a half hour later, but only dropped one package when I was expecting 2. I tried to catch him, but chalked it up as a loss for that day. About 15 minutes later, he came back and dropped the second package and was apologetic.
UPS seems to charge more, and they don't deliver Saturdays, which has cause me to get impatient plenty of times, having to wait all weekend for something to be delivered. USPS, at least the one in my town, is horrible. There's always a long line, usually one person, maybe 2 working the registers. If there's a huge line, there'll be employees doing stuff behind the counter, but not helping customers. They (now) tend to be higher or about the same as the others, but for the same cost, they claim it'll take a week compared to 3-4 days with the others. That, and their tracking is atrocious (pretty much non-existant). I hate going there, as I know I'll get ripped off every time. I'll drive a little further and use FedEx anyday.