Fyi: Take a look at the GAO's grim assessment of USPS's financial viability from earlier this year; here's the link:
http://www.gao.gov/products/GAO-09-475T
I agree with much of what has been said on this thread.
The two options, in my view, are, as they say, "apples and oranges." Postal carriers are still primarily letter carriers who have far less stress, far less risk of serious injury, greater opportunity for a balanced family life, and so on. UPS drivers relative to letter carriers are probably rather underpaid for the degree of difficulty of the work and the fairly high probabilty of disability.
On the other hand, UPS is a huge, increasingly varied, and financially strong company with a greater range of opportunities than the post office for those willing to work hard.