Generally it is very simple - the more expensive the raft the better it is, but the cost of the best can be awful expensive so depending upon your personal philosophy and laws/regulations in your country some folks opt for the cheaper units.
With a catamaran
you have two hulls that can provide flotation versus only one with a monohull
. With sufficient warps and stabilizing equipment
dragging behind, the survival success of staying with a catamaran
is much better than a monohull
which can roll in huge beam seas and flood/sink.
The big "*" (asterisk) on having a life raft is the annual or periodic inspections and re-certifying costs. There is no end of crooked or phony raft inspection/repacking business (at least on this side of the ocean) who fill your raft container with old magazines and trash and keep your raft at worst and fail to pack correctly and omit changing the survival equipment
accessories so the raft will not function when you need it. It is commonly recommended that you personally watch and oversee the inspection
and repacking of the raft with your own eyes. That way you see what you have and how it really works. After all, it is your butt that will be bobbing around in the raft, not the service/inspection company staff.