Neither. MacKeeper is considered malware - see Do Not Install MacKeeper - and Norton can have its problems as well. If you feel as if you need av software, stick with the free Sophos for Mac or just scan your system when you like with ClamXav. For an excellent article about Mac malware, see Thomas Reeds Mac Malware Guide.
If you feel that you must have AV software on a Mac, Thomas Reed suggests Sophos and ClamXav. Give Sophos a try first - unlike Norton, it's free. And it comes with recommendations from some of the real experts around here. I use ClamXav Sentry to scan, in real time, one folder - my Downloads folder. If you do decide to use ClamXav, get it from the developers page and not through the App Store.
This comment applies to malicious software ("malware") that is installed unintentionally by the victim of the attack. It does not apply to potentially harmful software, such as keystroke loggers, that may be installed deliberately by an attacker who has hands-on access to the victim's computer. That threat is in a different category, and there's no easy way to defend against it. If you have reason to suspect that you're the target of such an attack, you need expert help.
OS X versions 10.6.7 and later have built-in detection of known Mac malware in downloaded files. The recognition database is automatically updated once a day; however, you shouldn't rely on it, because the attackers are always at least a day ahead of the defenders. In most cases, there’s no benefit from any other automated protection against malware.
The most effective defense against malware is your own intelligence. All known malware on the Internet that affects a fully-updated installation of OS X 10.6 or later takes the form of trojans, which can only work if the victim is duped into running them. If you're smarter than the malware attacker thinks you are, you won't be duped. That means, primarily, that you never install software from an untrustworthy source. How do you know a source is untrustworthy?
- Any website that prompts you to install a “codec,” “plug-in,” or “certificate” that comes from that same site, or an unknown site, merely in order to use the site, is untrustworthy.
- A web operator who tells you that you have a “virus,” or that anything else is wrong with your computer, or that you have won a prize in a contest you never entered, is trying to commit a crime with you as the victim.
- “Cracked” copies of commercial software downloaded from a bittorrent are likely to be infected.
- Software with a corporate brand, such as Adobe Flash Player, must be downloaded directly from the developer’s website. No intermediary is acceptable.
Follow these guidelines, and you’ll be as safe from malware as you can reasonably be.
Never install any commercial "anti-virus" products for the Mac, as they all do more harm than good. If you need to be able to detect Windows malware in your files, use the free software ClamXav — nothing else.