The basic difference between Sirius/XM and HD is that Sirius is satellite
based, digital radio with quality comparable to CD's and content that is (mostly) commercial
free and unregulated by the FCC. There is a monthly fee and adapters are available that let you get Sirius through any existing home or car stereo.
HD radio is a new terrestrial based digital system aproved by the FCC and designed to bring radio stations (both AM and FM) into the digital age and allow them to compete more effectively against satellite
based systems. There are about 500 FM stations usig the system currently and a very few AM stations. There is VERY low consumer acceptance of the system at present and it requires a new radio to receive HD broadcasts. The big advantage is that once the investment is made...it is free. On the downside...you may not get better sound as many stations are using their digital bandwidth to broadcast MORE and different "channels" of music
to broaden their appeal. You also face the loss of signal with distance from the transmitter unlike Sirius which allows you to listen to a station anywhere in its' satellite footprint.
In my opinion, HD is a new and unproven system that is not a safe bet for cuirsers at this point. It MAY be a logical choice for homes in markets where HD is broadcast and where the benefits are obvious.
Sirius can be an excellent choice IF you can handle the monthly fees
and enjoy a wide variety of entertainment AND will be sailing within the "footprint" of the satellites which are optimised for land dwellers!
Here is the footprint...note the edges of coverage in the Caribe, Central America