I think it depends where you're sending it. I believe that sending things within the US, UPS is more expensive. However, with UPS, I believe they guarantee that the package will be delivered by a certain date, and they have tracking services that are included in the price.
When sending something international, in some cases, UPS (or other private companies like FedEx) might offer more value. For example, it might only cost a very small amount more than the postal service, but it is guaranteed to get there a lot faster.
I'm sure others here probably have more experience with this, but I have a friend who owns a Mail Boxes Etc. (actually, I think now it's called the UPS Store), and I recently had to ask him a question on how much it would mail something to Japan. In that situation, it only cost a few dollars more to ship it UPS, which he recommended.