G'day and welcome to the forums.
I would recommend using the AV in conjunction with Defense+.
The additional system overhead in initializing the AV engine, and its subsequent impact on system resources is negligible - I can't spot the difference at all.

Try it yourself. The AV component can be easily added or removed by re-running the installer.
Also, the alerts for the AV and Defense+ are designed to co-operate. If you get an alert for a known piece of malware, it is handled by the AV and you will not get a Defense+ alert. Depending on how you have configured the AV (you can supress the AV alert by setting it to auto-quarantine), it is possible to get NO ALERTS triggered by malware actions that would normally be triggered by Defense+, when the action is intercepted by the AV.
Hope this helps,
Ewen :-)