ok i can answer this
consept 300 is just a basic cat 1 alarm, which has a interior ultrasonic bubble sensor, and 2 immobalisers.
but you can add things like exterior, tilt and window smash sensors, also things like head light activation and auto window closeure, as well as intellistart, can start the car from inside your house.
it can also be set up to turn on heaters or radio etc ass well as door pops and boot pop, also a sensor under the bonnet so that if its opened when the alarm is set it will go off.
consept 500 is the same but u get blackjax (anti car hi jacking) duel zone proximity sensor.
if you want the best go for the avantgaurd but then u talking lots of money, but it comes with all the extras.
to do the boot release u need to take the boot buton out, then you can actually use the central locking motor that is in the boot to pop it, i can tell you how to do this if ya want.
go with clifford u wont regret it loads u can do etc, or go with toad but not as many features.