i use to work for ADT, and from what i can remember, ADT has 5 or 7 monitoring staions, that are linked together, in case "your" station goes down, it automaticaly switches over to the next monitoring station, atleast thats what they told us.
Brinks only has one monitoring station, again, thats what ADT told us.
9 out of 10 times all you really need is the sign in the front and back yard, most burglers will bypass your house for something easier. do your research. and go with what makes "you" feel safer.
another cheaper trick, if you have family or friends that have ADT, have them call ADT, and ask for more window stickers, they should just mail them some, and you can put them on your windows....