Here is my suggestion based on working in a UPS store for a very short time:
If the items have real value (ie documentable value where it costs $xxx to replace it, not "it's been in my family for 200 years and is priceless!" value) or need to be there by a specific date, use UPS. Despite having a PITA customer service, they still have guarantees and insurance (if you buy it). USPS is much more loosey goosey with handling the package and harder to deal with if there's an issue. Cost will be fairly similar, I believe. You can quote both out online if you want.