INMARSAT was originally designed for large ships. It required a fairly large
antenna and a terminal. The terminals have come down in size over the years along with the
antenna, especially the mini M, but it's still a lot of
hardware. Basically If you have this system it remains on the
boat. Another system is the ISAT phone.
Iridium is a handheld, like a large
cell phone. You can take it ashore with you with no problems. There is world wide coverage. Rates for both are about the same, and both offer data, fax, voice and very slow
internet. Speeds are equivalent except you can get higher speeds from INMARSAT at tremendous cost. You pay by the amount of
service you use, which is very pricey for both systems.
Some cruisers own Iridium phones and also
ssb radios. They do their
email over the
radio for free but call back to suppliers and
family on the phone. There used to be a
service that you could "rent" one for long passages, almost free if you didn't actually make a call. Many crossing over from
California to
Hawaii would do this in case of
emergency.