Registered is your safest way to ship. Over the years, as a Postmaster, I have dealt with many overnight packages that arrived late, arrived damaged, arrived without contents, or didn't arrive at all. Overall, First Class and Priority mail seems to be much more consistant than Express Mail.
For some reason, Express Mail seems to be more prone to theft while traveling through the processing plants than other classes of mail. Recently, I personally sent an Express item to Denver, and the package was opened in the Denver Express Mail facility and $600 in cash was stolen. Also, a month ago I sent an Express item to a small city that is around 110 miles from my home. The item was guaranteed overnight but it took 4 days to be delivered, and that was only after I called the Post Office and raised hell.
Registered is handed off from person to person, with each person signing for the registered item. The drawback is that because of the safer handling procedures, Registered is much slower than other classes of mail.
Yes, right now I am a pissed off former Postmaster.
Rick