If you are not concerned about passing a virus to someone else then you do not need either, at least for the time being.
If you are concerned about passing a virus to someone else then I would recommend ClamXav. It is free but donations are greatly appreciated.
When, not if, OS-X viruses appear can you imagine all the OS-X-non-anti-virus users who are going to get hammered?! Whew! Many people do not want to use AV software at all but for the rest of us it is simply not an issue if we are using a Mac with two, four or eight cores and ample RAM.
I wonder how much memory/processor time typical widgets use? Or the seemingly endless supply of totally useless software ***** people put on their Macs?
I like perspective.