really any need to get nasty?
there are benefits and problems with both systems wifi or gsm, as with pstn.
relying on one is okay until it fails when you need it most.
from my point of view wifi can be more problematic, but it will all depend on devices capabilities and what is going on in the vicinity.
gsm doesn't have to use your router to get the message out, so if something happens and there no power to the house, the wifi module will be working off the internal panel battery but your router without power back up can't send anything out, whereas the gsm can.