This is a normal situation of figuring out which type of backup is suitable for you. The principle of backup is, that any copy of data may disappear at any time. Then you need another copy (the backup) to recreate your lost copy. If you have 2 copies and looses 1, you only have 1 copy. In the time you only have 1 copy you are at serious risk of loosing your data.
Even if you have 10 copies of your data, there is always the risk of loosing them all. Sooner or later you will have to settle for what is good enough.
This is complicated by that different ways of storage have different way you can loose data. E.g. an external harddrive protects you against accidential deletion, viruses and failure of your primary drive etc. But not against fire, theft, flood and the likes.
A cloud backup like CrashPlan may loose your data or go bankrupt. There are examples of people who did not find out their backup was corrupted until they tried to restore. Remember to test your backup: If you have not tried to restore you may not be able to do it.
What I do:
I have a Synology NAS with RAID 1 (2 mirrored drives) and a backup using JottaCloud (a backup company based in Norway). I use CrashPlan for some other data.
I use RAID 1 because harddrives fail. A single external harddrive may go bad at any time. Never store data on just one harddrive if you want to keep it.