A search on CF for Iridium
will turn up a number of threads that directly address your question in infinitesimal detail.
1. Upfront cost is the same for Iridium
2. Iridium has recurring charges
3. Little or no learning
curve on the Iridium, big learning
curve on SSB
(just read the SSB threads.)
4. Iridium is more reliable, SSB performance is subject to time of day and atmospheric conditions
5. You will want compression software
to go with the satphone for downloading GRIBs
6. You can have preprogramed emergency
numbers in the satphone and this makes it easy to send out a help if your boat is burning or sinking.
7. You can take the sat phone in the dingy if your boat sinks. Can't do that with an SSB
8. Can call friends and relatives anytime and instantly by using the satphone like a cel phone.
Good luck on this subject!