A PLB (any PLB) will have a stronger transmitter, and thus has a greater likelihood of getting through tree cover, out of canyons, etc. So my feeling is that if it is one message that has to get through, I'd be more confident in a PLB. On the other hand, a SPOT can be used differently, if you use the tracking feature, because you can send a series of messages which show your progress, and thus if you suddenly have no progress for several days (which could be because you are too badly off to even operate the device), somebody would at lease know where to start looking. In that same situation with a PLB, if you can't use the device, nobody knows where you are or if you are in trouble.
On the third hand, though, if folks at home are counting on the messages, and one or more does not get through due to the device failing to connect, they can become worried and go through hell while you are fine (this happened to me with a first-generation SPOT).
My own choice would be the PLB.
Although for a one or two day adventure like you are contemplating, I might consider renting a satellite phone.