Guam is a U.S. Territory w/ one non-voting member of Congress. Saipan is part (the capital) of the Commonwealth of the Northern Marianas (Tinian, Rota, et.al.) w/ one non- voting member also. Saipan is "tax-exempt", but you pay approx. 7% of your income to the Commonwealth. U.S. Postal Service, and "some" U.S. laws apply. Remember the news a few years ago about the sweat shops there? They can import workers, pay them squat, and put a "Made in the U.S.A." on whatever they make. You pay tax on Guam, just like in the states. You file a 1040, and all of your schedules. The exact same tax laws apply. But, per an agreement with the U.S. Federal Government, all of the tax revenues collected in Guam, go to the Government of Guam. FYI, the rest of the Micronesian islands: Truk (Chuuk), Ponape, Yap, Koserae, et. al. became thier own country (The Federated States of Micronesia), with the capital in Ponape(Pohnpei, or however they are spelling it now). Just a few years ago, the last U.N. Trust Territory of Palau became it's own nation also. Note: all are served by the U.S. Post Office and have agreements in place for certain services with the U.S., including military staging.
I left Guam in '97 after spending eight years there w/ Continental Micronesia. At that time Saipan was served by:COAM, ANA, JAL, KAL, Asiana, United(since left),NWA,Phillipine Air,and Pacific Island Aviation. I really do not know who currently operates.