You may have the lucky or scary opportunity to select the cloud backup software for your company. Working at an MSP, I've had the opportunity to work with quite a few different backup programs in a variety of different environments.
First, the similarities. Both applications provide a clean, easy-to-use web console, a desktop application, and email alerts regarding backup status. You can backup external drives with either software.
Both systems back up the file structure - no gibberish restores with wonky file names. They also both provide some nuanced restore options - choosing files and dates is an option.
With Crashplan, you can backup to multiple locations at once using the tool - like a superpowered Teracopy. You can backup to Crashplan as well as to a local device, such as a NAS, backup server, or even an external hard drive. This is great for building redundancy.
Crashplan also provides stronger granulated incremental backups - you can restore a deleted file from 1 PM 8 days ago on Crashplan, while Backblaze allows only a limited amount of incremental selections.
The downside of Crashplan is that it is unreliable. I know that every time I check my email, I'll get the "hasn't backed up in ….days" email from Crashplan, despite the device being on and connected to the network the entire time. The Crashplan service needs to be restarted frequently (at least 1x a month in my experience) and doesn't always start on its own after a server reboot. Sometimes it never starts at all, as was my experience at one Mac-based office.
They pay the monthly fee and their devices never manage to connect to the Crashplan server. We switched them to a different service.
With Backblaze, there's no troubleshooting involved. It either works and is backing up the device, or the device is off or FUBAR. I've only ever had one device so busted up (software-wise) that Backblaze was incapable of running.
The backups are so reliable, it feels silly checking the backup alert emails, which always say that the data has been backed up recently.
Both have similar initial backup speeds. With Backblaze's new Business Groups edition, users can also create their own logins to the business account, a feature that Crashplan has had for a long time.
The granularity of Crashplan, ease and speed of the restore are some of my favorite features. Backblaze I love because it is a set it and forget it program - easy to install, easy to run.
Overall, I'd still pick Crashplan if I had the bandwidth to monitor it and resolve any backup issues. However, if I needed to focus my attention elsewhere, I'd definitely choose Backblaze - always better to have a backup!