So I poked around and found SPOT is using a group of LEO birds at 52 deg inclination. So they are horizon to horizon in less than 25 minutes. The complete cluster should provide constant coverage if your in the open. If you think about being in the bottom of a canyon and the satallite happen to flying directly over you might still only have a few minutes of good visability per pass. That is why the spot short message and multiple retries works. The "dumb" spot uses the older generation of satallites and the spot phone uses the newer higher bandwidth satallites.
When I drew my Utah Desert Sheep this year...I decided to buy the ACR unit...no annual contract.....peace of mind in your backpack...we are in remote Utah outback either hunting/hiking or on SxS machines alot...no cell coverage in most areas...after an ATV accident several years ago...a friend and I were first responders...accident victim really hurt bad.....took over two hours to get air med there...I believe the ACR unit could have saved a lot of time.... http://www.acrartex.com/
I don't have a need for one for hunting, But that's just Me. If it makes you feel more secure go for it.
Yes I have a 406 type ELT in the Plane I fly for work & it works very well. I had it go off one time shortly after we had it installed and before I could taxi in the Airforce had called the Airport for a crashed plane & our office in less than 5 minutes. I would not waste money on the old 121.5 type. Good & Safe Hunting.