Yes,
I agree that you'll want both eventually, in addition to Tongass Fjords and Southern Alaska. But for now if you can only do one and want to experience Return to Misty Moorings from it's base of operations, then Pacific Fjords is the choice you want to make.
Be sure to read the installation files for any RTMM scenery location you start with as they should all say which region the location is based inside of.
Brad