Inmarsat vs Irridium for Baja

[Bash]

The Risk Management Officer for the university where I work is requiring me to carry a sat phone on my Baja trip next month. My class will be sea kayaking, so this will need to be a handheld unit.

Please advise whether an Inmarsat or an Irridium system would be advantageous, especially if I end up renting the phone and service.