The regulation for fixed ELT mounts has been updated quite a few years ago. I cant remember the exact G loading that the base has to be but if you require easy to find out tomorrow and has to have an EO to fit it. Depends where you fly what you fly. The cost of a portable is much less but no inertia switching.
My advise have both. I carry my own elt with me in my tool kit, if im lucky enough to be able to get to it to use it I know I have it.
Also you have to register it with AMSA