From my experience often times UPS is cheaper than the postal service. Also, with USPS, "priority mail" is totally meaningless - not only do they force you to buy priority mail if your package is over a certain weight, but there is no difference between priority and "regular" mail. There's no guarantee your package will get anywhere, and if it doesn't arrive, there isn't really anyone you can even ask about it. My parents have lots of experience with UPS b/c of their business and have always been happy. Also, I like the idea of customers being able to track their UPS packages.
I have to disagree with a couple of the points above, though I have heard that UPS can be cheaper. As someone else mentioned, you can get tracking on your USPS packages -- and it's free if you print the label online.
I've noticed a big difference between Priority and regular first class in terms of time -- Priority is usually 1-3 days, and first class is like 3-6 or so. If you don't want Priority on a larger package, you can get parcel post which takes a little longer.
Also, if your package doesn't get there, you can do a loss report. I've only done it once, and they ended up finding the package based on the report.
(Hope I don't sound annoying about it! I think all of the different shipping methods have their place, and you should do whatever works best for your budget and customers!