The Vortex Venom & Viper are a popular micro red dot optics that I install on Glock slides and various other firearms. In my opinion the Venom is better in most cases since it fits standard sized Glock slides better than the wider Viper. In fact, the Venom is nearly identical to the very popular Burris FastFire 3.
The Viper however would be a better choice for the 45 caliber Glocks which have a much wider slide. Overall, both optics are very nice and well built.