UPS and USPS are both transport logistics. However, the ultimate speed of shipment depends on what you are shipping. If you have a small package shipment then USPS will work fastest. If your shipment is large, then UPS is a much better option. It is cheaper and gives you the opportunity to ship goods almost every day.
Talking about the reliability of the service, UPS works on schedule and delivers packages in the day it says. However, both services are reliant and the chances of losing packages is very small. Depending on your package needs, you can choose the best service for yourself.