most of the alarms are made by the same company, if not within the same warehouse type of deal. Dei has a large portion of the market with brands such as Clifford, Dei, hornet, viper, Avital and the list goes on. Audiovox has a good portion of the market as well, with brands such as Prestige, Audiovox, Code Alar, and more. They all are pretty much the same as far as connections go. You will get people to say "I like such and such better" but with things like the fcc and the restrictions placed on transmitters and the functionality of the units themselves, you really dont get any huge differences. Pretty much all alarms have a lifetime warranty.
With that said, I use Audiovox, Code alarms myself. I like The functionality. I like the remotes, they dont seem as cheesy to me as say the viper stuff. Theres really nothing you can do for the real thieves. But you can keep the honest ones out/away from your car as best you can. Things to annoy the criminal such as extra sirens, motion sensors, and blinking l.e.d.'s can help deter. Personally though, I like a nice clean, non noisy, non false alarming, system. I actually go as far as not installing led's and using the factory horn instead of the siren. Then again, I make sure that I am comfortable where and when I leave my vehicles.
There are some accessories like a pager system, or even tracking systems that you can view on line, in fact theres some neat features like emailing you or paging you when your vehicle has moved, or is speeding, or or or.. its pretty much endless what you can do, just got to figure out what you want from the system.