It depends really. Smaller packages will typically be cheaper with USPS. Also, check out the flatrate boxes... I forget how much they cost, but if you can fit 10 lbs of stuff in that box, it'll be cheaper than anything else.
But if you're shipping large boxes that are 5lbs or more, usually UPS and/or Fedex are cheaper.
Don't touch media mail with a 10 foot pole, though.