I just installed an Avital system in my 4Runner, and I'm happy with it so far.
One thing that needs clarification is the fact that almost every brand mentioned so far is designed, manufactured, and sold by the same company: Directed Electronics (DEI).
DEI product lines basically parallel each other, so if you're looking at say Python vs. Viper, there will be a model from Python that is roughly equivalent to another model from Viper, and another from Clifford, and another from Avital, and so on.
All the product lines use the same sensors (shock, tilt, audio, etc) and sirens, and equivalent products between product lines use almost identical control units (the brain of the system). The only major differences between the various product lines are the name, and the look/design of the remotes and antennas, which are the only parts that are visible to the end user.
DEI brands include: Viper, AutoStart, Clifford, AstroStart, AutoMate, Python, Avital, XpressKit, Valet, etc.