JNALLEN,
Damselfish is right, but let me expand on it just a little bit.
The Gekko is a very nice, basic computer... for a Sunnto. It still has a lot of features. The Viper is rather a premier unit except for wireless air-integrated models. (BTW, just to make sure you know, "air integrated" means that the computer monitors your tank pressure. Most computers aren't air integrated but this is changing as they become more reliable and affordable.)
Here are some differences to consider.
1. Gekko has no active backlight, you make it glow with a flashlight. Viper has a "real" backlight (like Indiglow watches.)
2. Gekko is cheaper. Viper has more gosh-wow factor.
3. Gekko can't upload dive info to your home computer, the Viper can do this. This is a cool/fun way to log your dives, but not at all a necessary thing.
4. Viper shows tank pressure, so no need to buy a separate gauge like with Gekko.
5. Viper can warn you of low tank pressure, Gekko can't. Viper can also tell you how long you can keep diving based on your current air comsumption - a thing that only air integrated computers are capable of.
6. Viper has "dive simulate" mode, Gekko only has "Plan" mode. Simulate mode is more detailed, and you can learn what the computer wants you to do and how the deco/no-deco times change as you pretend to be diving it.
7. Both are Nitrox capable, reliable, and safe computers. You would be well served with either one. If the money isn't a big issue I would get the Viper. It's a sweet little package and does have capabilities that the Gekko lacks.