WHM Backups are an good local backup solution especially if you plan on continue using WHM/cPanel. This makes it easier to migrate and restore user/domain databases, homedir data, and DNS (if you are using DNS on the WHM box) in case some bad happens or you need to migrate to another WHM/cPanel server.
Regarding disk space I always try to have double what I need and use half for backups. Preferably with backups having their own separate partition. If you need to save on space you can play with your retention, how often backups are ran, and compression.
WHM backups do have an option to let you FTP the cPanel backups to a remote location in WHM >> Backup >> Configure Backup. Rsync is a good option imo too even though cPanel recommends you stick with their tools in their FAQs.
Good stuff here: