Media Mail is generally cheaper than UPS. However, it is not traceable, and is considerably slower. Keep in mind that the USPS will subsidize some rates and penalize others. Media Mail has to be kept low for things like books for the blind, etc.
I have a hard time believing that there is some subsidizing going on. There still making money.
Postal Service says delivery and signature confirmation are available with media mail. http://www.usps.com/send/waystosendmail/senditwithintheus/mediamail.htm
It is definately slower. I ordered something via media mail awhile back and it took close to 4 weeks. I was in contact with the seller - I KNOW when they shipped. It was very strange. I really think USPS purposely slows it down just because it is media mail.
Another time I was the seller - it was a notebook of christmas crafts. The USPS clerk went over and over the description. I patiently disagreeded that a notebook was a BOOK ... finally I told her to open it, look at it, and reseal it in front of me. That's when she relented with a "well NEXT time... blah blah blah". What a pain in the neck. What should have been a VERY quick transaction took 15 minutes.