Capacity - What is the total capacity of the client's data?
Speed - Based on capacity and speed, you should make sure that your backup is finished during the after-hours window.
Media Lifetime - How long will the backup last on the shelf? Cheap CDs, for example, have a surprisingly short shelf life.
Durability - How much abuse can your backup media take? We never expect to drop an SSD or DVD, but accidents happen.
Form Factor Lifetime - Can your restore from a 100MB Zip Disc? How about an IDE drive? How long will you be able to restore from your backup?
Many of these considerations are becoming less critical with cloud backup, but they're no necessarily irrelevant.
Another major consideration is cost. This is particularly important if you rotate media out of circulation. No matter where you backup (including the cloud), you still need to have a complete backup that goes offline and is never over-written. For a 2 TB drive that might be $150. For other media the price will be different.
No matter what you use for backups, you absolutely need to understand how to restore from the backup. That means you need to truly understand the backup technology so you don't screw things up while trying to recover data. And, of course, you need to document the backup process and procedures.
LOTS more tips in my latest video: