Mailing Packages-UPS vs. USPS

This is something new to me. I have never had to mail my Christmas gifts before. I'm afraid I am not going to get my stuff off in time for it to arrive by Christmas. I also don't know which is better; UPS or the United States Post Office. It all seems expensive to me, so I really need to just get my stuff ready to go--still making some gifts!--and get them out! I guess whichever place I see first will be the postal service I choose.