Which one is better, they both seem comparable, although The VIper is slightly cheaper? ON ebay the viper can be found for around 220, compustar around 290.
I have the Viper and have been very happy with it. I looked at the Compustar too, I thought the remotes were nicer, however when I called a couple of "authorized" Compustar dealers in my area they told me they stopped selling them as they were problematic.
The other thing is, if you get the Compustar on eBay this voids the warranty (it says this on the Compustar web site). I don't know how the Viper warranty works on eBay since I got mine at Circuit City.
Although I am a fan of eBay I would really recommend that you get your alarm/remote start from an authorized dealer and let them install it, unless you know how to do it yourself. I think my Viper warranty says it needs to be installed by a pro for the lifetime warranty to be in effect. Also, if a part goes bad they will get it and replace it for free.
Circuit City, by the way, is not the greatest place to get this kind of stuff done unless you happen to know the installer who will be doing your system, maybe check with friends or co workers who have had alarms installed. I got really good recommendations from my local CC and that is why I went with them. They did a really nice clean install and it works perfectly.
Hope that helps you a little, like I say I don't know much about the Compustar but you cannot go wrong with the Viper.