It depends on what you are shipping. It also depends on if this is a one-time shipment or if you have a business that will be sending items out on a regular basis.
Personally, I ship via USPS because it's the most cost-efficient method for my business. I simply don't have the volume to set up a UPS or FedEx account. What really saves me money is that I have a postage meter at home, which allows me to weigh my shipments and print out postage online. If I print the postage through Paypal, I not only get the postage discount, I also get free delivery confirmation. If I purchase the delivery confirmation through my local post office, it's .80 extra.