In my opnion clifford is the best. I have an Avant Guard 4 installed in my mustang and it has been great. It offered all sorts of feature, Proximity Alarm, Tilt Alarm (prevents from jacking and stealing of wheels), glass break sensor, the Omni Sensor(a really nice impact sensor that actually works for the entire car, not just where the device is mounted, I could lightly tap the rear spoiler and it would recognize it) Battery backup for the alarm. Black Jax keypad ( each time you start the car enter a 2 digit code, if you don't enter code engine shuts off after 30 seconds, sounds like a pain, but not at all...optional usage) Remote Start, Roll Windows up and Down, Trunk Latch Release, turn headlights off and on when not in the car, and it has 1 or 2 programmable functions, one of mine controlled my neon in the car. Granted this alarm cost me about 1000 installed but it also comes with a $3000 anti theft guarantee. All clifford products are great and I will never choose another brand for an after market alarm.
I am also a fan of clifford because they are hard to disable. they hide the brain box up in your dash to prevent easy access. Then ontop of that every wire that comes out of the brain box is unlabeled and black, which prevents a thief from being able to locate a specific cable and cut it. I would go for the Clifford. Much better quality than the viper. Viper is pretty much the lower end of DEI while Clifford is the High