Both programs add the proactive malware detection, called SONAR, that is currently in the Norton 360 software. SONAR uses behavior-based analysis to attempt to identify new malicious software without a virus signature.
Both also add some network features. NAV will display a map of your home network, while Symantec says NIS will also check your wireless network security and offer network setup advice.
Norton Internet Security 2008 brings in a new Identity Safe, which Symantec says adds site password saving and support for OpenID logins to existing browsing security features.
Neither NAV nor NIS yet include technology from the recently announced Norton Antibot (also available as a public beta), a standalone supplemental security program that uses behavioral analysis. But I'd say it's a safe bet that Norton will add it to their product line much like they did with Norton Confidential.
You're asked to provide your e-mail address to download either the NAV or NIS beta, but after doing so you're taken directly to a download page and Symantec doesn't appear to validate the address. The NIS download is 62MB, while NAV is 56MB.