The Internet Security has more things than the Anti Virus. On top of the stuff included in the Anti Virus you get a fire wall, anti-spam, virtual keyboard to protect from keyloggers, anti-phishing, and about half a dozen other features. Worth the extra in my opinion.
However, I'm having issues with my computer right now which may or may not be a conflict between Kaspersky and Vista. Unfortunately I can't be more specific than that, it might be nothing, but I can't give it a unreserved thumbs up until I've used it a bit longer. Does get good revues though.