Fetch on auto-cron means that the emails are fetch whenever an agent is active.The normal cron just fetches email based on the settings you configured in the cron job.So when you've configured your cron job correctly, let's say it fetches every 3 minutes, it shall fetch every 3 minutes and you could safely disable the fetch on auto-cron option (since I wouldn't recommend having both, cron + auto-cron, turned on).