USPS First Class Mail is used to ship letters, envelopes, and lightweight packages. USPS Priority Mail is  used for shipments that are top priority and need to be shipped quickly. The services are not comparable, USPS First Class is about shipping and USPS Priority Mail is about delivery. Priority Mail is quicker and the weight limit is 70 lbs for all the packages. Thus it is also more expensive. Generally speaking packages under 16 oz should be shipped via First Class Mail, anything over 16 oz should be shipped with Priority Mail.