Full cron allows to run any command as cronjob on your server as web user, in this case web5. Its the least secure option.
Jailed cron runs the cronjob in the jail of the website, with this option you can only run commands that are installed inside the jail. Thats much betters ecurity wise.
URL cron allows only to run cronjobs by URL, it is the most secure option. ispconfig handles the url cron and creates the nescessary command in the cron files for it.