Audiovox is fine, its 90% installer. I work at best buy and i can tell you that we are 'supposed' to reccomend viper and in return they give us special accomodations (same w/ tons of other brands that give us special accomodations for recomending there products or saying "oh yeah, i have this and it works great" etc). That said, i personally do prefer dei because they seem to have a bit more support.
Just remember, when it comes down to it, if someone wants it bad enough an alarm is only a slow-down. Remember to get stuff like a backup battery (to prevent wire-snips or fender/battery drilling).