I've generally preferred USPS (UPS breaks my stuff ;^), but lately, things seem to be going downhill. Last month, I won a $1 auction on GunBroker - I mailed payment 8/23, and it took the USPS folks 16 days to get that envelope from Wyoming to Texas. The seller mailed my $1 part right away, but USPS tracking is showing the package orbitting down in Texas, moving in circles from the seller's location to Houston, and back again!
(EDIT: Finally received the above package 9/28, and the delay was NOT the fault of the USPS - the seller had shipped my part in a used box that had been mailed TO him, without removing all of the old USPS bar codes. The USPS scanners apparently kept reading the OLD bar codes, and routed the package accordingly, back to the sender. If you reuse packaging, MAKE SURE there are no bar codes showing!! )
About the same time, I bought a bunch of cast bullets from a seller in Arizona, and instead of sending it north (as mentioned, I'm in Wyoming), the USPS folks in Phoenix also sent
that box to Houston, where it's apparently been sitting without moving for several days (so much for "Priority Mail" ;^). Looks like there might be one of those "black holes" that
@Blackmore mentioned, down there in Houston, sucking in packages from all over the country!